if the tank is empty and has no fish or anything in it, iwould do NOTHING to try and speed it up, because youare cycling your tank and curing the lr at the same time
as for the time, it varies from tank to tank, and lr to lr, the best thing to do is monitor yur ammonia and nitrite levels, once they go high and then fallll on their own, back to ZERO, you will be done, time frame can be from 1 wek to 6 months(normally much less than 6 months and closer to 2 or 3 weeks, but possibly that long, in some cases)
jsut watch all your water parameters until it is finished, that is hte best advice i can give