In a healthy relationship it's normal to talk about things you're thinking of buying, so it would be a bit odd to make a major purchase without mentioning it to your partner, but there's also the issue about which 'pot' the money comes out of and whether or not the purchase affects the other person.
We split our money so we cover joint bills and savings and then each have equal amounts of spending money to spend on ourselves on how we see fit.
DP may or may not buy a new mountain bike costing several £k and the first I know of it will be when he asks if there is space on the credit card to pay for it, or whether I think he should look for an interest free deal because despite the cost being out of his spends, I have various schemes running to make extra bits of money from credit cards etc, or if it's a second hand one, he might need my help transferring the money to pay for it from the right account (he does have access to money but he's not interested and whereas I am interested, so look after the money and use it in the most advantageous way).
But I don't get involved in what could be months of research with him looking out for his N+1 - the looking is part of his hobby and then if he sees a deal he'll go for it, while I wouldn't even think of looking for a new thing until I actually needed it.
And I do often get a new car without consulting him, as I have a company car and I just order a new one that suits me when the time comes, and the first he knows about it is when I say 'the new car is coming in X weeks' although if I decided to opt out of the scheme, which I'm currently thinking of doing, I would discuss with him as he would want to do a lot of the maintenance himself, so I'd want his opinion on something he is happy for me to have, because I just want a reliable car that doesn't cost too much to run and is comfortable so as long as those boxes are ticked, I wouldn't care whether it was brand X or Y and if he said that a certain car had expensive tyres or known common faults, I'd want to take that into account in my choice.
But our own smaller purchases out of our own spending money, we don't generally discuss and I don't think putting a number on it is helpful, because you could have one person who spends little day to day, but makes the odd large purchase and may be spending less overall than the person who's constantly buying lots of cheaper bits and pieces.