Full amount is advised if you want a return like savings, and even then you may just be unlucky.
I’ve only had a small amount in them for most my life. Finally got a £25 and then a few months later another. Then nothing.
Most of my savings are elsewhere, generally fixed rate ISAs though I must get round to shifting them about as each expires. Still not a great return. I have one in stocks & shares but I should have cashed it in at peak, all gone down again. Still it’s had the biggest return having doubled the investment, but that’s taken 20 years!
Housing on the other hand 😉