Let your gut read on your tenants help to guide you in this. Maybe as little as once a year or as often as once a month. More frequency reduces risk, as long as it is not so often that you are interfering with your tenants` enjoyment of their home.
But ultimately make this an element of your management system. So maybe every 2 or 3 months you stop in to replace the furnace filter, check the humidifier, etc. Every spring you may need to co-ordinate a spring cleanup (you don`t need to do the work but you could be there to decide what is to be done). Every fall you need to do some winterization maintenance (shut off the outside hose bibs etc).