Which particle decays are quickest? 
The $ρ^0$ and $\pi^0$ mesons have the same quark content, (uu*-dd*)/sqrt(2) and have masses of 775 MeV/c^2 and 135 MeV/c^2 respectively. The ρ0 has a lifetime ~ 10^-24s while pi0 has a lifetiem of ~10^-16s. Explain the difference in lifetime?

I think the ρ0 decays to pi+, pi- pair and the pi0 decays to two photons but how do i know which is quickest (other than being told so in the question)? More generally how do you know which particle decays are quicker and why?
 A: Have a look here, on the four fundamental forces and ther typical interaction times.

The couplings are directly responsible for how fast a decay happens, bar some conservation rules for special cases. 
The classification happened because of observing decay time differences similar to the  difference between rho and pi0, and modelling mathematically the decays.
Decays happen when there exist lower energy particles to which  an unstable particle can decay. The decay time is hierarchical, if hadrons are available, it is the strong decay that dominates the probability of decay, if quantum numbers allow it, the electromagnetic comes next. The weak appears when conservation of charge or other quantum numbers inhibits the electromagnetic. The gravitational one is very weak and not involved in particle decays.
The rho because it decays to two other hadrons has the characteristic strong decay. The pion in general, has the muon to decay to, so the charged ones go to a muon and muon antineutrino. There is no neutral muon, so the pi0 goes electromagnetically to two gammas.
