When should I start the eviction process for a tenant with consistent late payments?

I have a tenant who pays half of the rent at the beginning of the month and then the other half at the end of the month, without a valid excuse or reason. I have allowed them to do this for the past six months, and I haven’t really received a valid excuse or reason why it’s happening besides a simple “sorry.” The unit is free-market, and I haven’t raised the rent since they moved in back in 2021. At what point do I tell them I’ve had enough and start the eviction process? I make all necessary repairs as soon as possible, too.