http://babywhispererforums.com/index.php?topic=64158.0Check out the link...gives approximate times. For 3 months its says its about 1 hour 20 to 1 hour 30. Whatever your LO can manage!
At 3.5 months some activities we did were:
1. Looked at a few toys one at a time. She liked to just lie and stare at them (Sulley from Monster Inc, a butterfly, a rattle).
2. I read her very simple books (Each peach pear plum, Goodnight moon, Babys ABC), I know it sounds mad but she loved them and now she will sit and listen to books for AGES and will independently look through books on her own. Great for being out and about.
3. Singing songs! LOTS!!
4. Musical toys (her favourite was a wind up tv that played a tune while the picture scrolled and the mobile in her crib)
5. Sitting in her chair (had three wee toys that hung down)
Hope thats helpful!!