When I get close to a bonk, a gel lasts me 5-7 miles. I prefer munching on Fig Newtons and four of them (or similar stuff, like one Trader Joe's blueberry bars) will get me another 20 miles or so.
To put it another way, I think of a gel as a way to get the last few miles to a SAG stop. Fig Newtons are part of a day-long, unsupported ride eating plan.