If I remember right, the i2 charges at 500mA. If the HG2 s were completely discharged that would be roughly 6 hours, give or take a bit extra for any losses, as the process isn't completely efficient. If they're not completely spent, it'll take less time.
My Luc V4 is a godsend when charging these. It'll charge at 2000mA (2A), so a hour and a half, if I really need them quickly. I prefer to charge at 1A though if I'm not in a hurry. Nothing wrong with a lower charging current though, it takes longer but will have a (very minor in the scheme of things) positive effect on wear and tear.