My daughter started a Montessori program for 3 days a week just before her second birthday. This program lasted all day (8:30 -2:30) with an afternoon nap time. By the end of her second year (when she was about 3 1/2) she was asking all the time to go to school everyday. So this year, just before she turned 4, she started going all day. While I definitely can tell a difference in her energy level now that she's going 5 days, she still seems to have the energy to do many activities of her own choosing after school. We don't have her enrolled in any other organized activities at the moment though.
Since your program is only 2 hours, 45 minutes, I think a 3 1/2 year old would probably enjoy it every day. Also, I noticed when my daughter started going to school every day, her sleep schedule seemed to even out. Previously, she would sleep in on mornings when there was no school, and her bed time was more erratic. Now, she goes to bed more willingly and, I think, sleeps better than she did when her schedule wasn't as regular.
Good luck with your decision!