I think a even a single day is enough. If I post a patch one day and
don't commit it until the next (or even later that day), I always either
do an update or fresh checkout/apply patch and build/make check to
ensure nothing has interacted with what I am applying. Good lord, I
think it is simply appalling to check in patches which are 2 weeks old
without testing them again.
For major check-ins, I usually do a fresh checkout and build/test cycle
afterwards to make sure nothing broke. (or make sure I didn't forget to
checkin a file, etc)
You cant reasonably remove the couple of hours window between applying
and building/testing/checking in, but it at least vastly improves the
chances of not causing problems. The person doing the check-in should
exercise due diligence to ensure they didn't introduce anything
catastrophic.