My road commuter has just gone through its chain in 9 months and they def last longer (as they dont get so muddy or as much torque) so I would say that an average used MTB chain esp in muddy condition prob won’t last more than 6 months?
What I use when I can’t remove the chain with a quick link.
Cleaning and using lube regularly will increase the length of wear however I would be more concerned with the longevity of the chain set – if you can eak more time out of the chain set by replacing the chain then all the better. Some very cheap chains will wear quickly – what chains are you using?