About 3-5 days is the typical shelf life for wheat paste (until it begins fermenting and smelling icky) which is also about the time your posters will be getting plastered over. Using a paint roller or brush from a hardware shop is usually the best method for putting up posters or artwork. Lay on a basic coat of wheatpaste using a paint roller and allow it to dry.
It s a good idea to soak your paper pieces in wheatpaste before utilizing them in paper mache projects. It will take around an hour or two for the glue to set but the paper will remain moist for at least a day. The procedure can be sped up by using a fan.