With mortar between two almost-impervious surfaces, I'd wait a couple of days. If you need to walk across a traffic area, lay a sheet of plywood over it the next day to walk on. (At $60-70 a sheet, I'd not wear shoes when walking on it, and I'd return it soon after.
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The larger the tile, the longer the drying time.
And if you happen to be using a light-colored grout, you'll want to use white mortar instead of gray.