If you have koni's those are supposed to be forever will let the guys that have had them on the bus for a whole bunch of miles be the judge of that I don't have that data.
If u use the Monroe gas shocks on the front or those front gas from prevo the black ones they will get u 20,000 miles. If you have a straight axle and you don't have four shocks up front that's definitely an upgrade that you want to do really really noticeable Improvement on the straight axle going to four shocks. Koni for independent front suspension is what most guys get and set them for firm not soft if u do. Even though their lifetime I don't know if I'd want to keep them in there for longer than 10 years but maybe you can.
On the rear you could do it a couple different ways correctly in my opinion. If your budget can afford it put koni back there if not go with the yellow Monroe gas Matic shocks they're only about 60 bucks a piece. you can go 50,000 miles on them on the rear and that's a great value and those are really good shocks.