We didn't have cell phones for a long time because we didn't need them and wanted to save the money. Finally got Verizon 2 years ago last month. I really, really like my phone number, it's kinda cool, so I'm wanting to hold on to it as long as possible.
We had the same landline number for 44 years. When we dropped our landline and went with cellphones only, the change was painless. Send out some emails, called our Luddite friends and gave them our new numbers. Made all the necessary changes at banks, doctors, etc. Kept the landline for a month after we said we were dropping it, just to be sure we didn't miss any important calls. No calls. No complaints from people who couldn't reach us. Learned a valuable lesson from that. With all the smartphones and speed dialing and voice dialing, not too many dial all ten digits manually when calling. Changing a phone number is not that big a deal. It certainly shouldn't prevent one from changing services, or getting a new "local" number if that is important.
I rarely dial anyone's number anymore, and don't remember most folks' numbers. Just find 'em on the contact list and hit the phone icon.
But to answer the OP's original question, we've had our same cell numbers for 5 years.