The gorge of Samaria is the second most visited tourist attraction on Crete (following the Minoan palace of Knossos) and by far the most popular walk. More than a quarter million people walk through the gorge every year from May to October. In winter the gorge is closed to visitors due to the danger posed by water and falling stones.
If you would like to avoid the crowds, you could choose to start your walk towards the middle of the day. This way you could reach Agia Roumeli in the evening, having enjoyed the gorge all to yourself. You could spend the night in Agia Roumeli and then have a wonderful day at the beach before you leave.
Nevertheless beware: the Samaria gorge is not just an easy stroll. The path is 16km long and although it is maintained and in good condition, it is quite stony and also steep at times. If you are not used to this type of exercise you may find it pretty heavy going and will certainly feel your legs for days afterwards.