Lots of Time
I am developing something myself (hush hush, it'll definately be a front pager). Graphics are a BITCH! you did a really good job on yours though. The gameplay is a nah.
Notes:
To scroll, have the camera follow the character (pm me, I can give u a script)
Have the character when character starts and stops moving a couple frames of acc/dec-eleration.
You should have a ranged weapon (again, pm me for script)
GREAT Graphics! Work on the gameplay and you're all set.