Well, if your charger can charge your lipo at 1C (1C charge rate = 8000/1000 = 8amps), it would take 40-45min to an hour depending on how discharged the lipo was.

What is your charger's max charge rate. 50watts won't be a prob time wise with 2S lipos. If your charger's max charge rate is 5amps, a rough guess would be about 30% longer than time expected with 1C charge rate.