Taking properties Sub-2 - How long for loan payoff?

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I'd be interested in hearing from members who have purchased or sold properties subject-to existing mortgages.  What's the longest time you've kept the existing lien in place?  What agreements did you make as buyer or seller?

I've only used sub2 as a buying and problem solving strategy and never kept one open longer than a year, and never used sub2 for a buy and hold.  I put in my purchase agreement when the final payoff on the loan will be. 

I'm curious what sellers are agreeing to. Being relieved of payments on the property may be motivating to some sellers.  But three years down the road, many sellers will be looking to buy again and/or will be annoyed that loans in their name are still open.  

Experiences?