Depends entirely on what you're blanching. For example, if it's Swiss chard, or kale, I'd bring the water to the boil, add the veg then, as soon as the water comes back to a boil, wait 30 seconds, drain, refresh, ready.
Green beans I tend to leave about 2 minutes (and I always use the timer on the stove); broccoli, cauliflower, peas, the same. You mentioned snow peas - same as chard or kale.
Things like carrots, parsnip, potatoes, beetroot, artichokes, aubergine (eggplant), pumpkin, courgettes (zucchini) don't get blanched here. They're cooked from scratch to table.