Assuming you mean the last 20+ mile run before the race, many plans will have a 3 week taper, so you'll be running arond 20-22 miles exactly 3 weeks prior to race day, then a little less the following week, then less again, then race week.
So it might look like: 20, 16, 12, 26.2
A three-week taper is the most common i've come across but 2 weeks are perhaps more common for elites, or more experienced runners, or some might prefer 4 week taper, but to me that seems a bit too long and there's the risk you'll be "stale" by race day. Some people will have done 3, 4 or 5 x 20 miles in their marathon prep, whilst someone following a "get round the course" plan with a lower level of initial fitness might not have been able to manage more than 1 or 2 x 20milers in their prep.
Basically there's no "one size fits all" approach, but i'll be doing a 3 week taper, and i think given it's your first marathon that might be about right for you too. it's far enough out for you to recover in time for race day and be in top shape on the start line, but not so far that you'll start losing the training benefit of all those weeks of prep. Good luck
