The number of weeks to run Alpha will depend on how many videos you show on the Alpha Weekend. If you show 4 videos on your Alpha weekend, here is an average for each series:
Using the Alpha Film Series - 11 weeks
Using Alpha Youth Series - 9 weeks
Using Alpha with Nicky Gumbel series - 11 weeks
If you show only 3 videos on your Alpha weekend, you'll need to add an extra week to your Alpha (either before or after your weekend).