>>>I will make a note of contacting you (off-line) when the time comes that I won't be able to support my vertical.
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>>A bad point in time to start looking. Do it at least 3 years in advance as to not have to bargain under pressure and have more options to make transition a tandem work effort.
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>All depends how many customers I will still have by the "point" when I need to find a replacement.

Actually, your replacement may need several months just to get to know the way you did things, where's what in the code, what's the app doing etc. Specially in a vertical, which may get very rich in features. The last one like that took me years to get to most of the stuff, and there are still areas in it which I never visited, let alone touched. And working with a partner may get you not just ideas on how to improve the app and keep up with the times, but may actually bring more customers. So I'd take Tamar's 3 years seriously.