Hi,
I'm certainly no expert in the field of silicone but I believe I read on their website that A4L guarantee their aquariums for 10 years, I'm also sure I read somewhere that it will last much longer if kept permanently underwater rather than a a wet dry scenario.
Personally though once a tank has reached 5 years of age I'll start looking towards buying a new one, for the sake of say £500 it's not worth it for me to ruin everything in my room along with deluging the shop with water that I live above.
Having said all that, there aren't that many stories at all on the web of tanks letting go rather than the glass cracking from not being level although it does occasionally happen.
Cheers!