I was curious about something I've got a 1996 nissan sentra gxe with the ga16de motor in it. What the average life expectancy of the motor if it is maintained pretty good?

That's not a fair question. You can't ask for "average life expectancy" if something is "maintained pretty good."
Normal maintenance can take these motors a very long way. I've heard stories of over 300,000 miles and still running.
Are you going to use it to commute? Plan to tow anything? Idle a lot with the A/C on? Too many questions to predict any actual life expectancy since it ultimately comes down to asking what is the weakest component in the entire engine and trying to keep all of the pieces working together...

If you use SAE 5W-30 full synthetic engine oil, and change it and the filter every 6,000 miles or 6 months, whichever comes first, and change the coolant (50/50 mixture of manufacturer recommended antifreeze and distilled water) every 30,000 miles, the engine should last 300,000 miles. Since most people don't take care of their transmissions, they never see 300,000 miles. ATXs can go the distance, if they are very well maintained.

As for the maintenance I get the oil changed every 2000 miles instead of 3 because with my car the engine seems to get sluggish after 2000 miles, I put about 25 miles a day on it, I hardly use the ac, it's never been in the shop for any majore repairs, I've replaced the starter twice, alternator twice and the cv halfshafts and the brakes a couple of times and the other fluids at the recommended intervals

How many miles on it now? Changing oil at 2,000 is wasting money. I'd do what TGM suggests.

251,000 and running good

You are well past life expectancy, enjoy the moment.

I don't think I'll have any trouble out of it for a while but I do keep having exhaust leaks and any thoughts on cutting out the resonator?