Technically, most of the airlines have 3 to 5 years Miles credit limit unless you keep making 1 to 3 transaction with them per year that prevents miles from expiring.
Some Airlines, that is irrelevant to a point that your miles would expire irrespective of your transaction unless you hold a status with them.
For theoretical purpose, You want an airline that provides you lifetime guarantee of the miles. However, even if your miles were guaranteed for the life, the value of miles changes almost year by year.
Airlines such as JAL started almost with $5 per mile value and has currently dropped to almost 90 cents. So even if you preserved your mile forever, you won’t be getting much value out of them. Even with Jet Airways, use of miles have dropped drastically. It went from requiring 6000 JPmiles for upgrade from economy on Mumbai-Delhi route to 10k Plus JPmiles.
Even the features of use of mile is depreciating. Earlier upgrading from miles was as sure as done deal. Now, desk agents ask you to wait for a while till they close the counter for the particular flight. They upgrade you at very last moment. They want to provide customers with cash upgrade option first vs someone who wants to use the miles.
It is always smarter to earn and burn your miles within 2 years span as beyond that, they lose value drastically.