Most CC companies have a 90 day limit on charges that can be contested. Amex is the only one that I know of that goes further back (1 year IIRC but it may be longer).
Stripe has decided to change that and will refund the $15 if you win. It's a nice change but in my experience the vendor rarely reverses chargebacks.
I did some brief searching and it seems like the time limit depends on several things (including type and amount of charge and physical location of the vendor). It could be as little as 45 days or as long as 540.
It's definitely become more common to get chargebacks over the years. Part of this is increased volume on our part and part of it is the CC companies becoming more consumer friendly.
I think it depends on the chargeback reason code. If the chargeback was due to fraudulent reasons, merchants have very little chance of having it reversed. Now, they may be able to fight it and win based on procedural reasons relating to the chargeback dispute process. Not something worth doing though for most merchants.
Sometimes, its more costly in time to fight a chargeback than the fee that is imposed. Many merchants just let it go and eat the costs.