You can make the rollover at a later time without a problem. If you are rolling from a 401k to a Roth IRA, the timing will affect your taxes. If you are rolling from a 401k to a Traditional IRA, also called a Rollover IRA in this case, you can do it at any time.
The only exception would be if you have less than
$5000 vested in the 401k they can force you to take a distribution from the plan. From experience, not all plans enforce this provision, but they are not forced to keep your 401k open unless you have at least $5000.
I found out some additional information recently that I wasn't aware of.
Apparently you once the money is in a Rollover IRA, it can be converted into a Roth IRA at any later point without incurring a distribution penalty (of course, you still have to pay that taxes at the point of conversion).
There is no time limit for the conversion either.