(Please note Lombre evolves with the Water Stone and not the leaf stone)
I'd personally say evolve at level 19. Lombre learns Fake Out at that level and the rest of the level-up learnset is either underwhelming, or comes too late.
EDIT: Fury Swipes, (25) Water Sport, (31) Thief, (37) and Uproar (43) aren't usefull. Hydro Pump (49) comes too late.