Seedling mangos will bloom and bear in three to six years.

http://www.crfg.org/pubs/ff/mango.html

Here is a link that might be useful: Mango cultivation

Jag2010, its really not a "height" thing. When the tree is ready it will bloom. Mine took appx. a year to bloom after I bought it, and another year for it to "hold the fruits". On the other side of the coin, I've seen people buy the tree and it blooms for them a few weeks or months after they get it. But if the tree isn't strong enough to hold the fruits its best to let the tree mature and get a little bigger before you let it fruit. A little patience is a good idea...although that's a virtue that's very hard for me,lol.

Here's mine with blooms a year or so after I bought it.