Are you saying you already prepared some Roxies in a solution for injection in a syringe, but now you are wanting to do some coke instead and save the Roxies for later? And you are wondering how long you can save the oxycodone/water in the syringe?
I would say it depends on how sterile your preparation was. Oxycodone is pretty stabile in water, it's not going to rapidly lose potency or something, the thing you have to worry about is bacteria growing in the solution, which can cause infections. Can't really give you a time limit. When were you planning on using it? Tomorrow or something?