Way too long. The first version didn't make use of webgl so it ran like molasses. Then I started looking into webgl, but it's a total nightmare so I ended up using a library called PixiJS which legitimately made things a million times easier. Took me like 2 weeks on-and-off.
Protip: if you're making an HTML5 game, make sure it uses (or defualts to) webgl. Let something like PixiJS take care of the blood sacrifices and satanic rituals that webgl requires. Or better yet use a framework like Phaser (which uses PixiJS) so you don't have to reinvent a dozen wheels, even if you think it's overkill.