I have some previous answers that might be helfpul:
How long would it take to sail from Venice to Alexandria during the Late Middle Ages?
It's hard to give an "average" time because there really was no such thing as an "average" crusade, the circumstances were different for all of them. But basically, it took a few months by foot (although it took much longer for the First Crusade because they kept stopping along the way), and by ship it took 5-6 weeks if you started off in the Mediterranean.