I had given it a massage shortly beforehand, but I had Azurill evolve at level 19 - just a level away from Marill learning Aqua Tail. :) Not planned; simple dumb luck. (This is relevant - Azurill and Riolu evolve the same way, except that Riolu must evolve during the day and Azurill cares not. They also have the same base happiness.)
Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere
It mainly depends on how much time will you spend walking with him. Mine did evelve around level 17-19 while fainting once or twice. My Leavanny on the other hand evolved on level 21 just after evolving into Swadloon. Almost same thing with my Globat (i cathed him on level 15 to be exact). It's just take a few more rounds around Castelia City with the bike.