When I used to use As5 I had to reapply at least every year. it does breakdown/seperate. Since then I switched to Arctic Cooling MX2. It's super thick which I actually dont like much( it's a pain to apply) but, it has no cure or setup time like As5 and they claim it's life after application is 8 years.
From my experience with AS5 and watercooling you're talking months. You probably want to re-apply every 6 months at most for optimal cooling.
Personally I think all AS products are overrated. Shin Etsu is reckoned to be the shit these days, though I find it hard to get in the UK so I tend to go with Arctic Ceramique which works well enough for me.
People don't seem to realise that mounting pressure and evenness of mount count way more than the TIM used.