How much difference in net length is ok?
The more precise terms to use are setup time and hold time, since your clocking scheme may vary. That is, your signals may be sampled on both edges (DDR), and so on.

From what I understand, setup time is for how long your data needs to be stable before a sampling clock edge and hold time is after your clock edge.



With a falling-setup and rising-sample scheme you practically make both these times \$T/2\$. Given your clock frequency of \$50MHz\$, the period \$T = \frac{1}{50\times10^6} = 20ns\$, thus \$T/2 = 10ns\$.

According to this answer the worst case propagation speed through copper is \$5.3 ns/m\$. Plugging this into the thing gives you \$\tilde{}1.9m\$. This if of course totally wrong, thus I must have wronged something. Someone please correct me.