There is no formula that I know of. Since you can only charge at 5/8 of the 1C rate of your 8000mah, that will necessarily make it take longer. Decent estimate would be 80min (1hr + 1/3more time ~ 80-85min).

Lipo uses a 2 stage process - the 1st piece is at constant current (CC) - the 1C rate. Then when the cells reach max cell voltage (4.2v/cell), charger switches to constant voltage (CV) holding the max total voltage on the lipo - for a 2S that would be 8.4v. This stage tops off the lipo and you'll seee current gradually decrease to near zero where charge will terminate. This stage takes most of the time and is affected by the lipos internal resistance. The nature of the CC/CV charge is why there is no formula for calculating time.

I just know from experience with lipo that at 1C a lipo discharged to the lvc will take about an hour. I rarely run down to the lvc and shut down when I see my RC starting to slow down, that takes about 40-45min to re-charge.

There is a 'fast' lipo charge option in some chargers and that option just eliminates the CV portion of the standard charge. You get about 90% charge in less than half the normal time. The other way to speed things up is to charge at higher than 1C - not an option in your case.