This is one of those questions that you almost cannot answer!!!!!!
I 'think' you will know when your brush needs to be changed It depends of a few factors but first and foremost how you look after it.
Obviously if it clogs up and you cannot clear it!!!! Or if it is contaminated....you can tell this if it starts to yellow the product. (Wipe your brush, after soaking in the monomer, on a clean white tissue and if it is tinged with yellow it is probably contaminated)
I remember asking this question once and got a bit of a vague reply :lol:
I would say you need to change your brush when it effects the standard of nails you are producing. I was told that brushes can last up to 6 months, but i think if you look after your brush then it will look after you and last quite a long time. When i was first doing nails i seemed to need a brush every couple of months as i kept wrecking them due to not rinsing in monomer after use. So they get clogged easily. I also use a bigger brush than i use to and just find as my technique gets better, i use my tools better and they last longer. HTH's.