[5th_bike]

Mine said to charge until the charging current goes down to 0.3A. With my 1A charger that took about 6-7 hours, so you with a 2A charger, should be good with 3-3½ hours. [note: edit/additions below]

[Edit:] Check the instructions to see if they mention an charging amperage with the specified 2 hours charging time.

If they mean 2 hours with a 4A charger, it should be 4 hours with your 2A charger then.
If they mean 2 hours with a 2A charger - follow the instructions.