4 years, basically.
Nexus 6 was supported for all security updates from launch in November 2014 to this month (November 2018). In terms of feature updates, it got up to Android 7.1.1, and I am not aware of any major app that wouldn't run on that today. Not exactly great, but certainly not a new phone per year. I believe Apple's policy is 5 years, rather than 4, with a reported risk of iOS updates purposefully making the original hardware performance worse (e.g. battery life).
Strictly as a geeky side note, if you have the time to spend DIYing your primary mobile OS (which, admittedly, very few people would prefer doing), there are pretty cool third-party Android distros and other OSes that will keep a Nexus 4 (2012, 6 years old) alive and well. Possibly a Galaxy Nexus (2011, 7 years old) too, but I am less sure about that...
Edit: To clarify, what I am saying is that, for most users, iOS' long term support is better than Android's. But, for Google phones and the like, it is only slightly better, not day-and-night better.
Easier said than done, especially when looking for more obscure apps. Also, it's not easy for non-technical folks to tell the difference...
> OS fragmentation: doesn't apply if you're not a developer, just pick a phone that will have a reasonable amount of updates over its lifetime
How long is "reasonable" relative to iOS? I recently moved from Android to iOS mainly because of this.
> Privacy protection: really depends how you feel about Google on this one
I'm not sure feelings are relevant here. The truth is simply the truth.
> Presence of a secure element: this one really depends on your threat profile
Agreed, but it's good to have the extra security guarantees.