Usually once a year, but that's only my phone. I replace my MacBooks usually every 1.5-2 years and I just got my first iPad so I'll have to see when I'll replace that or if I'll even buy another one because Apple devices hardly ever break unless you really try so it's not that you'd actually need to replace them.
Not too often. I have had the phone I am using now for about a year and a half. Its an Iphone 6 and I recently cracked the screen up so bad, but its not a big deal to me. I still use it like normal lol.
I can see myself upgrade after 3 to 4 years. I don't really bother with the higher end smartphones at this point though. Maybe eventually, but not now, and besides I couldn't really use the higher end phones for a cheaper prepaid pay as you go plan as I just don't talk too often on the phone thanks to shit like social media. I find myself texting more often on the phone than talking. And having a data plan doesn't really matter to me at this point since I'm busy with work and school so no real need for data unless I'm going to actually travel frequently.
That and also they quickly go obsolete after 3 to 4 years anyway. But I'm starting to think it's starting to reach most of it's peak at this point.
I also have not decided to get any tablets yet so I do not know. I've been thinking about the iPad Pro and the Surface Pro 4 or the Galaxy Tablet, but have not been able to decide which one I absolutely will purchase yet. I think they would be great for school since they are more lightweight and I could just store and read e-book versions of textbooks onto tablets. Also easier to take notes with a tablet with the right note taking software programs or apps.