How long can I keep stock from prawn shells in the fridge

I find this question interesting- it seems to me that this sort of stock would lack the fast-decaying proteins that make seafoods go bad so rapidly (note- this is an assumption; they may go bad for some reason I'm not seeing); it might store fairly well. Personally, I've always made it for immediate use, but I used to freeze the shells sometimes- they were good for several weeks.

For seafood, I would only keep it 2 days.

I wouldn't keep it in the fridge more than two days. Freezer? That depends upon how you freeze it. Frozen, it could keep up to three months.

I think you should just freeze it. safer bet unless you plan on using it very soon.

I'd say 3 days no problem. 4-5 days maaybe, esp if you bring it to a rolling boil before incorporating the stock into whatever you're making. Or freeze the stock in an ice cube tray and avail yourself of concern :)