As long as it takes.
I'm used to build single models (D&D and such) and I underestimated the time investment in painting an entire army.
I got the most of it second hand and it was a pain to remove the mouldlines and sprue-nubs from the models. It takes so much longer than doing it part-by-part.
I'm not building a Helbrute, got it new, and there are parts that are a pain to clean, like the spine. But a pointy scalpel does the job. It just takes a bit of time and attention.