No problem, 2 days will be fine, you would know if it was useless as it goes very watery.

How long does wallpaper paste keep?

Wallpaper paste will last for at least 48 hours, meaning you can return to papering the next day without having to mix fresh paste. Cover your paste mix with cling film or seal with a plastic bag, making sure that it's airtight. When you're ready to use it again, just give it a good stir, and it will be as good as new.

How do you know if wallpaper paste is bad?

If the paste is too diluted and fluid then it will have decreased adhesive properties and could cause blisters under the wallpaper. If it is too thick then it could cause lumps beneath the surface of the paper.