If the property is putting money into you pocket, with income exceeding holding costs by a comfortable margin, then you can hold indefinitely and wait for the right time in the market cycle to sell and capture your appreciation.  

Is there a stipulation of time frame or could this go on indefinite?

Here's the analysis I've done:Reasons to sell:Incredible ROI already ($200k+) just on market appreciation aloneMarket appreciation unsustainable (12+% YoY is wild, and Boston is now more expensive than pre-2008 Bubble prices)In a few years, further appreciation could allow me to be debt-free (this condo could likely pay for a single family home in the burbs with no debt, with all of my extra income going into a REIT or passive index fund rather than another property)Reasons to keep and rent out indefinitely (20+ years):If I put it on the market right today, based on confident, conservative comps for rent, as well as my real-world expenses that I know would stay consistent (i.e.

She would have the right to re-new her lease indefinitely.

If you have capital gains outside an IRA you can use the 1031 exchange to defer taxes indefinitely.  

The tenant wants to stay on for an indefinite period.

Every municipality has varying degrees of flexibility in how rigid zoning is enforced.Part of your pitch might go along the lines of 2 house, 2 loans, 2 families living there, etc - but 1 property.  2 Of everything will continue indefinitely (assuming it is legal as is) and all you want to do is clean up the current situation so each family can own their own property.  

Price and rent amount considerations notwithstanding, if you are agreeing that a tenant gets to stay in the property indefinitely, I would make some stipulations on her tenancy. 

His deposit will show pending indefinitely.

Aaron right now rent control only exists on existing tenants and it can be charged more....But after November election if proposition 10 passes they might be able to cap rents universally and indefinitely.