I buy throughout the year....mainly smallish bits and bobs, and then it's one big present for Xmas and one big present for birthdays.
It's a bit miserable I think, if your child is really keen on something, and will use it, and it's a 'good toy' that will last, to not maximise the pleasure by getting it when their interest is at its peak. If mine are being good, save a bit of money towards it, do a few chores etc, then I'll usually buy the toy they want after a week or so.
They know it's not every blimmin week though and they don't exploit it. They are 8, 14, and 17, now, though I've pretty much always done this. And for babies and toddlers I refreshed stuff often, usually via the charity shop or eBay.
All my DC are winter birthdays, so now we often do a family present for the summer holidays eg swingball, new board game, a new bike (usually secondhand as we've got a fab bike shop nearby that does reconditioned ones).
(I am on a tight budget btw, so I still need to save to do this, and I do say no to quite a lot, esp. Pound shop rubbishy toys).