I got my first FP1 in January 2014 and am still using my second FP1 (I forgot the first one in a train). So I have been using a FP1 for 4 years and 8 months (excl. 2 months last summer after I had lost that phone).
What makes me hopeful to use it for some more years is that we have spare parts here in Vienna, that were donated by other Fairphoners. Several other #fairphoneangels also keep a spare part stock. This, of course, applies to FP1 in my case, but even when Fairphone cannot sell spare parts for the FP2 anymore, the community will step in and distribute spare parts.
Note that Fairphone will still be able to sell FP2 spare parts in two years and hopefully after that because when Fairphone founder Bas recently visited Vienna, he told us that they had secured spare parts for at least 3 years and that he is aiming for 5 years.
PS.: What I’d like to add: If you are feeling nerdy, you could get a second-hand or refurbished smartphone that is well supported by the aftermarket operating system LineageOS, for example a OnePlus One, Samsung Galaxy S5 or Google Nexus 5 (according to the statistics I assume that many installations -> many developers working on it -> fast updates). A second-hand/refurbished phone is even more sustainable than a FP2 because it doesn’t use up original resources. The counter-argument is of course that you would have to manually install LineageOS on a second-hand/refurbished smartphone (I linked instructions above so that you can learn more about the necessary steps).