Depends on individual and outgoing mine has 2-4k I am not aggressively filling my ISA's at the moment so it might as well sit there.
So even at an astronomical interest rate of something like 3% that's still not even really worth more than half an hour of time shopping around to get a better rate.
Well I am derisking and taking money out of my PtP other wise I could get 4% with zopa
I keep 3 months pay as emergency cash on hand in that account.