"Is there any defacto standard interest rate recommended for landlords to charge tenants that are trying to catch up up their rent?"

So you think you're some kind of bank?

No; there is no legal "defacto standard interest rate recommended for landlords to charge tenants that are trying to catch up with their rent?"

In fact, since you've accepted rent since this event it might be argued in court you can't charge a late fee for something that happened earlier this year.

Either agree to work with your tenant or begin the process of eviction.

Gail