I wouldn't hold my breath for an updated iPad Mini anytime soon; the last update happened 2.5 years ago (in September 2015) and there were some rumors last year suggesting that Apple might be discontinuing the iPad Mini line.
So if it's absolutely a Mini you want, I'd say go for it now, but do be warned that the current iPad Mini isn't really a great deal (at least for the price that Apple sells them) compared to the normal iPad line. It only has an A8 which wouldn't be a huge step-up from your iPad Air performance wise (which has an A7 I believe), and the $399 it sells for isn't so great when you consider that the most recent iPad starts at $329 and yet features the faster A9 processor and other benefits over the Mini. There are also strong rumors about an update to the iPad line later this month which would make the gap even larger.