How often can I write on the flash chip?

Flash devices can be written to, at minimum, anywhere between 100,000 and 1,000,000 times (according to the manufacturers). Keep in mind we have a normal distribution and also, that Flash never ever just stops working, but merely distinct blocks do. I.e. you won't be able to write to them any longer, but you should be able to still read them.

For details see Flash memory