Depends how often you wash your hair and what colour you use; I wash mine every other day and the turquoize and the pink have been the most permanent of the ones I’ve used, but generally the darker pigments stay the longest (midnight blue is also very strong).

If you pastelize the colour or use a weaker colour (eg. grey, orange, lilac) then it tends to wash out a little quicker. Overall it usually takes me about four weeks to need/want a top up on the dye, but even then it’s never been completely washed out, and the times I bleach to start over it’s because my roots have grown through rather than the colour being faded.