first of all you have to keep in mind that the warranty for agx's is void if you lower more than 1.5". how long will they last will depend on what springs do you plan on matching them with? if you're staying with the stock springs or something that will not lower more than 1.5" and dont have some insane spring rates they will probably last as long a set of OEM shocks/struts.
i had a set of agx's on my 91gsx along with B&G springs that lowered the car 1.6" all around and the spring rates were not crazy either. the fronts blew in less than a year (around 8k miles) and one of the rear ones was leaking but didnt fell like it was blown.
on my 91 galant i also had agx's and ground control coilover sleeves. the previous owner installed them right before he sold the car to me.
not even 5k miles later the fronts were shot to hell and the adjustment was stuck on one of them. i'm assuming they blew up quicker because of the high spring rates of the GC. both cars were daily drivers and never road raced or autoxed, except the ocassional drag race.
most of the time i had them set at 1 front and 3 rear. but on weekends i was setting them at 2 front and 4 rear. hope this helps.