Split the movement of the projectile into two parts - horizontal and vertical speeds.
Then just use the vertical velocity as that's the only bit that matters.
You could work out the height of a projectile at any given moment by applying the vertical climb and subtracting gravity.
Obviously gravity will be -10m/s after 1 sec, -20m/s after 2 secs etc.
When the height reaches 0 then that's how long it stays in the air.
Then I'd just store these results in a quick look up table and work out the vertical velocity on the fly from the angle.
This will work fine for games but I'd suggest something more rigorous if you're planing on launching men into space
Mike