If you get it from a reputable source like TAP plastics and build your own it should last over a decade-- my lab group builds a lot of acrylic tanks for oceanography experiments, and we have tanks going back 15-20 years that are still fine. It's pretty easy to build your own if you want, local stores like TAP will cut to size for free so you just have to glue it together/seal it. I do my sealing with acrylic glue to keep the optical quality the best because my experiments are all video analysis & I'm avoiding glare, but normal silicone sealant is cheap / easy to apply if you don't feel comfortable with acrylic glue.
My personal aquarium is glass as a stop-gap, but I'm planning a fun hand crafted one out of acrylic in the near future. I'm using good local acrylic though, so I don't know about commercial tanks. The scratch resistance/optical quality can widely vary by acrylic type + coating, so if the company is using crummy plastic then you'll have a yellowing issue. Or if they have a bad coating- we got a bad batch of acrylic a while back that supposedly had a scratch resistant coating and it quickly degraded into poor optical quality.
So I guess I'd try to find reviews from people who own the tank brand(s) you're considering purchasing? Here's where I punt back to the forum--anyone have one of these?
