How much (%) of your mining income (or profit) goes back into expanding/replacing mining capacity? I know mining is for people with money but I wonder what's the smallest investment that can actually be self-sustaining in mining?

Myself I have free hosting (paid in bulk by another business of mine). but even so, the total returns on my mining are not enough to get something more than an S7 after several months, an S9 at the moment would mean putting some of my own money into this..again. So I wonder how much I should set aside (10, 50%, 100%?) to replace old hardware like S4,S5.