The amount of time required for drying will depend on the adhesive and its composition of materials. Normally, it will take about an hour or less before it bonds, but there are also others that require a whole day before it dries. You will need to apply pressure on both surfaces to make sure they bond well together, and there is no need to redo it. The number of cans you need for reattaching the headliner will depend on the restoration project itself. However, you would usually need two to three cans if it is a full-size headliner.