chances are you could work unhindered for ages as its pretty common on bases for people to spend ages working on cars after theyve come back from time deployed to find it wont start

You wouldnt get far if you came over fence as these days they usually have cameras and sensors in them.

Your best bet is coming in on foot with a fake id card and enter the base on an entrance where the walkway is furthest from the guard post. Most guard posts are in centre of road so they can check vehicle passes for expiry!!

You just need the right clothing and chances are they wont see pass any closer than a few feet away!