I generally wait a few weeks. Well no. Actually when I decide I "need" to buy something (yeh right - like "need" is the correct word) I wait about 10 seconds. But and here is the but. In the interests of me convincing myself I am being serious and responsible I go on the internet, research, agonize and reflect on it for a week or so or maybe a few months even (depending on the price involved and nature of the purchase). I try to convince myself I really really did need to make the purchase whereas the actual decision was made 10 seconds after the thought came into my head. I have never yet actually been successful in convincing myself of that, instead I realize now that I am just papering over the cracks in my psyche. Still, I can play the game.
It's the realization that we are all just weak, impulsive, irresponsible children wanting Christmas to come around. But hey that's human nature.
The wife does it with dresses. I do it with cameras. In fact we even play the same games - I buy some camera gear and sneak it into the house. I keep it out of her sight for a few weeks, sneaking away into my study now and then, to see it till she is suspicious and half convinced I have a secret lover I am emailing or phoning. Then eventually I casually bring it out. When she inevitably asks "Is that new". I can reply with a half clear conscience "No I have had it for ages". She is so relieved I am not having an affair she forgives me. She does the same with dresses. We both know the other is lying through their teeth but it keeps the peace in the house - a kind of mutually assured destruction if I can phrase it that way. Or a kind of domestic balance of terror if you will.
Seriously I usually mull over big purchases for some weeks or more. But I have been known to walk into a camera store, fall in love with something I like and buy it without thought. I kind of liken the former approach to the approach taken when deciding to marry. The latter approach is like a drunken decision to have a one night stand.
