@sun it only takes 2-3 weeks of eating extreme amounts of lactose to build a gut microflora that is able to succesfully digest any and all lactose you can possible shove in it
just 15 to 20 days of diahhrea, for never having diahhrea again (assuming no antibiotics + assuming drinking milk a few times a week forever into the future, so like 3+ liters a week, spread out, alternative milk products also count as long as they have more than 0 lactose in em)

you could do this with a single christmas+newyears time vacation away from work