With my BlackBerry's, I would upgrade with every new Bold line, so 9000 > 9700 > 9780 > 9900 so at least one year between upgrades and in between I tried out the Torch sliders but returned them within the return window as I couldn't justify keeping them.
After getting my Bold 9900 and deciding to try out Android over the last two years it essentially went: Bold 9900 > Galaxy S2 > Bold 9900 + Galaxy S2 > Bold 9900 + Nexus 4 > Z10 + Nexus 4 > Z30 + Nexus 4 > Nexus 5.
Say what you want about BB's, I've never had any trouble selling them where I am (i.e. less than 2 days from posting) and I have always gotten a good price for them (never losing more than $100 off the list price). The N4 on the other hand was a complete PITA to sell. Enough that I think I'll give my N5 away rather than going through that again.
So, yes, I'm taking a hit but I'm recouping enough $$ that it helps justify each new purchase. And its not like the phones aren't sitting in a drawer and I'm forking out full price for every new phone as that, for me at least, wouldn't be possible.
But I admit that with the N5, I don't think I will need to upgrade anytime soon and I'll see how long I can get by with just having an Android phone. I tried with the S2 and it didn't last and I tried to make the N4 my primary phone but that didn't work out either. Since I don't have a BB to fall back on I really don't have a choice but you never know... all depends on what comes out.