J.S.
If you feel your child needs an expanded social environment, Montessori is a great choice and it sounds like you have good feelings about the school. I taught Montessori for a while and definitely saw a pattern wherein little ones were really stoked the first week, then unsure the next, then soon found their niche/rhythm/way in the room and did fine. A short, upbeat goodbye and a mellow welcome home from you will go a long way.
If you do choose to reduce or limit how much time your child spends at the school, a shorter day (no less than 3 hours) for a cluster of consecutive days (not every other day as then everyday is a transition) can work well for a little one. However, if you want her to really feel like the classroom is family, doing more than 3 days a week is probably key.
The school (other parents, administrator, etc) will probably have some great tips for you as well.
Good luck :)