Hearthpwn

It depends on merc rarity and the bounty (it seems droprates differ slightly). In general it goes like this:

-You need 2700 coins to max out most mercs (3 skills 5 levels each and 3 equipments with 4 levels. Some mercs have less equipment to max, e.g. Sylvanas has only 3 levels on her deathrattle equipment, whatever it is called)

-Old tasks gives 985 coins to specific merc and some amount to random ones (I'm to lazy to count the exact amount. Those coins are nice early on but since we still don't have duplicate protection they become worhtless later on).  New tasks (for merc that unlock one equipment throught bounty rather than task 7) give 55 more coins, 1040 total.

-As such with the merc already unlock and tasks done you still need 1660-1715 coins. 

-In case of legendary mercs whom seems to be most consistent in terms of bounty reward, you get min. of 15 coins from heroic bounty, rarely 20 and even more rarely an extra pile. I would say that it's rougly 20 coins of average per bounty. For common and epic mercs coins piles seems to be a bit bigger (20 is quite common, sometimes you can get 25).

-Finally 1660/20 is 83 - bounties to do (86 for 1715) . It a bit hard to tell how long it takes to finish a bounty even on average (it depends on you skill, avaible mercs, treasures and the bounty itself) but I would say 15-20 min at least in my case. As such maxing a legendary merc takes 21-29 hours with no extra coins from tasks, packs or ranked rewards (and shop of course).