If you raise rent enough to get a long term tenant to move you’re probably looking at 2 to 3 years before you break even at the higher rent value. It’s almost certain that you’re going to go at least a month without collecting rent and you’re also looking at some deferred maintenance costs (carpet, paint, old appliances, etc). Ideally you want the tenant to stay but pay a higher rent value. It’s really tough to say what too much is in that scenario. Certainly as you approach true market rent the tenants are more and more likely to look at what’s available. If they find something shiny and new for roughly the same price they might elect to leave.