Hi Charly
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 don't think there is a set time. HTH.
Love Nikki xx