How soon after sealing grout can you use a shower?
Manufacturer directions should state minimum time after sealing for use.  It is important to “test” sealer by sprinkling a few drops of water on the sealed grout to make sure it is repelling water. If the water is spreading out and being absorbed, you need to apply another coat of sealer and allow to dry at least two hours. 

Here are the instructions on the Mapei container: