"Coherence length" is not necessarily a clearly defined term. A laser source can have phase jitter but still have a very large coherence length, as long as the phase jitter is periodic or very small. A laser whose cavity has a fixed given length may emit multiple wavelengths corresponding to different longitudinal modes, resulting in what appears to be a relatively short coherence length of a few centimeters. However, in a two-path interferometer such as a Mach-Zehnder interferometer, it is easily seen that the fringe contrast (a measure of coherence) is a slowly decaying periodic function of twice the laser's cavity length.
A bit of thought reveals that the absolute coherence length of the laser (the path length difference at which fringe contrast drops to 50%) really depends on the Q of the cavity: the average number of times a photon bounces between the two mirrors of the cavity before it makes its way through one of the mirrors. The longer a typical photon stays in the cavity, the longer the laser's absolute coherence length.
A laser's coherence length can be increased at least two ways: by increasing the reflectivity of the cavity mirrors (to increase the Q), and by increasing the length of the cavity. In graduate school I was involved in a project in which a laser was constructed with a kilometer-length cavity. (It was used to monitor stretching of the Earth's surface.) We never measured its coherence length, but its Q was probably around 100, which would have given it a coherence length on the order of tens of kilometers.
But if coherence length is defined in terms of fringe contrast in an interferometer with unequal path lengths, there is another way to increase coherence length: by phase locking. Two separate lasers can be made mutually coherent by interfering their beams and continuously adjusting the cavity length of one of the lasers to maintain a nearly-zero phase difference between the two beams. The frequency of the pair of lasers may drift, but the difference between their frequencies - and their phases - will be nearly zero.
If the two beams come from the same laser, but in the interferometer one beam traverses a very long path before being combined with the other beam, it is possible to continuously adjust the laser cavity to maintain a zero phase difference, even if there is a slow or periodic drift in the laser's emission frequency. But the combined beam will always produce high contrast fringes in a downstream interferometer, as long as the path length difference in the second interferometer matches that in the first interferometer.
The combined beam from such a laser will behave as if it is emitted by a laser whose cavity length is equal to the difference D between the two paths in the first interferometer: the coherence will be a slowly decaying periodic function of D which can stretch out to many times D. The decay rate of the periodic function depends on how close to zero the phase difference between the two paths can be maintained (in practice, better than 0.00001 radians).
This means that it is possible (in principle) to make a laser whose coherence length is light-years long, in that the phase difference between the light emitted now and the light emitted years ago from the same laser is maintained at zero. In practice, the long path in the first interferometer can be down a coiled length of optical fiber, as long as the attenuation coefficient is small enough that a reliable signal can be received at the other end of the fiber. The lowest attenuation coefficient of an optical fiber is around 0.22 dB/km, which means that a fiber can be as long as 100 km and still conduct a reliable signal. So, a coherence length (per this definition) of around 1000 km is readily achievable.