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Case (1) You keep the He sufficiently cold as to always have liquid He present.

The vapor pressure depends on the temperature as follows for He 4. He I and He II in the figure indicate the two liquid phases of He 4. A pressure of 1 atmosphere is about $10^5$ Pa. Above the critical point there is no distinction between the liquid and gas states, so you need to keep the temperature below about 5 K to maintain any liquid He. Note that at these low temperatures (below 5 K), any air/water vapor trapped in the top of the container will not contribute to the pressure.

Updated response: Case (2) If the He is at room temperature it is all gas. You can estimate the pressure using the ideal gas law $pV = nRT$. You know the temperature and volume, and you calculate the moles n from the initial mass of liquid He that has all evaporated; then you calculate pressure p. For a more accurate estimate, not assuming an ideal gas, you need to have a table with the state properties of He at room temperature. The total pressure will be the partial pressure of air/water vapor plus the partial pressure of He gas.

He cannot be a liquid at room temperature. You would have to have very cold liquid He to start with then let the temperature increase and evaporate the He in the container. I assume the container is not initially completely filled with He. (If completely filled, as the liquid He heats up it could rupture the container, since the density decreases with temperature and a liquid is essentially incompressible.)

Case (1) You keep the He sufficiently cold as to always have liquid He present.

The vapor pressure depends on the temperature as follows for He 4. He I and He II in the figure indicate the two liquid phases of He 4. A pressure of 1 atmosphere is about $10^5$ Pa. Above the critical point there is no distinction between the liquid and gas states, so you need to keep the temperature below about 5 K to maintain any liquid He. Note that at these low temperatures (below 5 K), any air/water vapor trapped in the top of the container will not contribute to the pressure.

Updated response: Case (2) If the He is at room temperature it is all gas. You can estimate the pressure using the ideal gas law $pV = nRT$. You know the temperature and volume, and you calculate the moles n from the initial mass of liquid He that has all evaporated; then you calculate pressure p. For a more accurate estimate, not assuming an ideal gas, you need to have a table with the state properties of He at room temperature. The total pressure will be the partial pressure of air/water vapor plus the partial pressure of He gas.

Based on your question I assume you will keep the He sufficiently cold as to always have liquid He present.

The vapor pressure depends on the temperature as follows for He 4. He I and He II in the figure indicate the two liquid phases of He 4. A pressure of 1 atmosphere is about $10^5$ Pa. Above the critical point there is no distinction between the liquid and gas states, so you need to keep the temperature below about 5 K to maintain any liquid He. Note that at these low temperatures (below 5 K), any air/water vapor trapped in the top of the container will not contribute to the pressure.

Case (1) You keep the He sufficiently cold as to always have liquid He present.

The vapor pressure depends on the temperature as follows for He 4. He I and He II in the figure indicate the two liquid phases of He 4. A pressure of 1 atmosphere is about $10^5$ Pa. Above the critical point there is no distinction between the liquid and gas states, so you need to keep the temperature below about 5 K to maintain any liquid He. Note that at these low temperatures (below 5 K), any air/water vapor trapped in the top of the container will not contribute to the pressure.

Updated response: Case (2) If the He is at room temperature it is all gas. You can estimate the pressure using the ideal gas law $pV = nRT$. You know the temperature and volume, and you calculate the moles n from the initial mass of liquid He that has all evaporated; then you calculate pressure p. For a more accurate estimate, not assuming an ideal gas, you need to have a table with the state properties of He at room temperature. The total pressure will be the partial pressure of air/water vapor plus the partial pressure of He gas.

Based on your question I assume you will keep the He sufficiently cold as to always have liquid He present.

The vapor pressure depends on the temperature as follows for He 4. He I and He II in the figure indicate the two liquid phases of He 4. A pressure of 1 atmosphere is about $10^5$ Pa. Above the critical point there is no distinction between the liquid and gas states, so you need to keep the temperature below about 5 K to maintain any liquid He.

Based on your question I assume you will keep the He sufficiently cold as to always have liquid He present.

The vapor pressure depends on the temperature as follows for He 4. He I and He II in the figure indicate the two liquid phases of He 4. A pressure of 1 atmosphere is about $10^5$ Pa. Above the critical point there is no distinction between the liquid and gas states, so you need to keep the temperature below about 5 K to maintain any liquid He. Note that at these low temperatures (below 5 K), any air/water vapor trapped in the top of the container will not contribute to the pressure.