I have just re-newed grout (in my less than one year old shower installed by builders) that on close examination had slight cracks but never the less water got through and soaked my bedroom carpet behind my wardrobe over a 4 month period. I used Mapei grout which was recommended. How long should I wait before using the shower and is there something else I should apply to make sure this does not happen again. Any advice would be appreciated.
If possible wait 24 hours before allowing water to hit the grout. On the bag there should be a defined time such as 6-8 hours dry time. I would advise getting some grout sealer to apply to the grout after 72 hours. Just curious what type of grout did you use sanded or unsanded? Was it pre-mixed or mix with water?
Sealing grout is not waterproofing.
The wall behind the shower should have a waterproof membrane or barrier. It either does not or has failed. Sealing the grout may buy you a LITTLE time but water will get through the grout again. you may already have significant damage, rot, mold in the wall cavity and any structure below the wall.
There are plenty of options. I used grout and tile sealer when I redid my tile shower in November (re-grouted exactly like you did), and can tell you it seems to work. Basically just spray it on and wait for it to dry.