This sort of information should be included in your article, too. It's an obvious hole in your article and you should probably have thought of it when you were writing it.
Long enough to discover that I have sausage fingers. Long enough to realize that they did a great job with the automatic corrections, but that when that fails, the process of correcting a word is brutal, and that it happens enough to make the whole experience grating when you're used to a keypad. Long enough to discover that there's no way I could shoot off a text one-handed without looking like I can on a device with a dedicated keypad. Long enough to know Blackberry-esque models are for me.
I don't know how many minutes if that's what you're asking. I don't think it's a hole in what is clearly an opinion piece that I didn't describe the exact length of time I spent typing on one.
This sort of information should be included in your article, too. It's an obvious hole in your article and you should probably have thought of it when you were writing it.