Yes, you can walk on them straight away. Your carpets will only be slightly damp after cleaning. You can walk on them as long as your shoes are clean. However, for the very best results restrict unnecessary traffic until the carpet is dry.
Be careful when you walk from carpeted areas onto hard floors eg. Tiles, lino, timber etc as this can be very slippery. Keep your feet flat and take small steps. You could also place a white towel or drop sheet to step onto as you come off of the carpeted area [be careful using towels that are coloured as there could be some dye transfer].