Depends a lot on if you think you’ll need large sums of money ready to go, and if you’re worried about inflation.
If you’re like me and routinely buy huge values and volumes of goods to make profit off of via market arbitrage, you’ll want a lot of ISK on standby to make those purchases, but the rest can be assets that you think will appreciate in value over time, to not only serve as an investment but also to protect against inflation, which slowly makes your ISK worth less than it was before. Do note however that no investment in eve is certain to give a return, and a good strategy may be to invest in multiple items to reduce the chance of getting ruined by 1 item losing all its value.
If, on the other hand, you don’t need to have much liquid ISK on hand and just make ISK in ways that don’t scale with liquid ISK, like abyssals, incursions, most pve, you can invest much more into assets you think will go up in value.
All in all, it’s down to how much ISK you wanna have available to use immediately, how worried you are about inflation, and how well you think you can guess what investments will pay off or atleast retain value over inflation. If you don’t care about inflation and don’t trust yourself to make wise investments, you can hold onto as much liquid ISK as you want. Otherwise, I’d suggest finding something you think will rise in value over time.
Remember, NO investment is certain to work well. TONS of people got burned hard when PLEX started falling in value throughout 2020, and now it’s down massively since its peak.