It's a slightly long-ish, but very doable ride from town (pretty popular club ride), or you can drive somewhere further out of town and then ride. I've never ridden it straight from Corvallis (done it either from the south near Monroe or from the opposite side instead), but I'd guess from OSU (which is near the edge of town going that way) it's maybe a 50-60mi round trip depending on whether you just stick to the Hwy or try to take less-busy roads around Philomath. Total elev gain is probably 3500' or so, of which maybe ~2300 is the 'main' climb from the hwy turn-off approx 9-10 miles from the top, where there's an improved viewpoint and a couple public cans. If you do the math, avg grade of Mary's Peak Rd is probably something like 5%, which doesn't sound too bad, but there's a pronounced 'dip' about 1/2-2/3 of the way up and it slacks off near the summit plateau so if you subtract those there's several miles of 6-7%.

It's certainly less than Mt Lemmon (although MP gets steeper), Mt Palomar, McKenzie Pass, Hurricane Ridge, etc, but it hosts a hillclimb race every year which has also rotates as the state hillclimb championship some years. For being so close to town, it's also a great place to just go hiking and such, even a little snowshoeing in winter without having to drive another hour+ each way to the Cascades.