Zero - I've got some quite strong filters at both the ISP and local email client level that "delete from server" for known dross.
Something that has worked well for me for about 15 years is to have one of my own domains used for "purchases/memberships/subscrptions/enquiries" where I seed the address - e.g. if I buy something from Bloggs Products I will use the email address bloggsprods@<my domain> just using the before @ as a seed* etc. The incoming mail has a "catch-all" forward so the messages come direct to my "real" email address. Thus I can set up filters on the "from" email addy to either delete from server or direct them to a folder on my email client.
That's in addition to other filters that pick up specific words - and either delete or junk folder them.
[Some stuff beats the filters in which case I set up a specific forward to piss-off (at ) aol.com (obv the@ symbol) and the incoming message flies off into the ether!]
Once it's set up it contains the email dross and my incoming mails are kept at zero unread.
At the other end of the scale my email client (Time & Chaos Intellect) has probably 7 or 8 years of filed emails - classified by topic - useful to find stuff I've bought or whatever.
* this also, of course, identifies situations where someone has passed on or sold my email address to a "trusted partner"