I didn't read all the replies in detail but I have a general rule of thumb that I use for myself. It's actually pretty simple (and somewhat flexible depending on factors).
The basics are this. When I look to sell a property I look at what I expect to make over the next 5-7 years. I try my best to factor in things like future repairs (if I think a big expense is coming soon, like a roof, HVAC replacement, etc is coming, that factors in too), future rent increases, etc. If the money I can make after taxes (let's ignore the possibility of a 1031 exchange for now since that's a whole other conversation, but of course is a consideration) when selling exceeds what I can expect to make in a 5-7 year period, I usually sell. I had one property that I could make ~$50k after taxes if I sold and it was only making me about $300/month in cash flow ($18k - $25k over 5-7 years). So roughly a 14 year period overall if I didn't sell. Even factoring in future appreciation and rent increases, maybe 10 years at best. So I sold. I can turn that profit into more than $300/month cash flow with new properties, hence increasing my overall cash flow.
I do think about other items like future appreciation (if it's potentially huge, I may hold off and sell later), the area where I am in (If it's an area I don't plan on being in the future or at present, that factors into the "sell now" column), etc. Bottom line is I try and take it all into consideration and barring anything gregarious, I can take the cash and use it to increase my cash flow now (which is my goal - highest cash flow possible as soon as possible).
So with my goal of creating the highest possible cash flow as soon as possible, that process usually works well for me. Also, to be clear, I've been doing this a while and thus far, this decision has only even been thought about for properties I've own for 5+ years. It usually takes mortgage paydown + good appreciation before I even think about selling a property (because of my goal of cash flow above all else).
And most of the time, I actually consider taking the cash-out refinance option for more properties vs selling. I usually sell when those factors like big expenses coming, getting out of an area I no longer want to be in are more prevalent. When it comes to cash-out refinancing vs selling, if I like the property and it's in an area where I'm happy, cash-out refinancing is usually the way I go (being tax-deferred and all).