I try to do preschool at home most days with the girls (Lena, born 2010, and Maggie, born 2012), but there are lots of days when it just doesn't happen, and that is perfectly fine with me! When they are officially school age, we'll have to comply with required hours and attendance and all that. For now, though, I like to get them out and do as much as we can. We do preschool at home on the days that we're home, but we spend a lot of time out in the world.
We do our preschool based on weekly themes. I've found that it's easier to plan when I've got a basic framework or topic on which to build. I have a list of weekly themes, and have them mapped out for the year on a calendar. I spend an hour or two the week before, or over the weekend, planning out activities for the week.
Our schedule varies widely each week based on appointments, holidays, the weather, scheduled play dates with friends, and the availability of my mother and mother-in-law who come to help some days. Becasue of this, I don't even attempt to schedule when we will do what for preschool. I just start the week with my ideas, supplies, and resources ready and see what I can fit in during appropriate times. I keep my planning page taped up above my desk, in a very visable spot so that I can easily see it to choose what we can do, based on the girls' moods, energy levels, and my energy levels.
Even on days we are busy, I try to spend 20-30 minutes on our preschool theme. Sometimes that is just reading during breakfast or playing outside. If we are home all day, I may spend several hours on preschool.
Overall, we are heavy on the books and crafts, and heavy on the imaginative play. They'll have plenty of time later on to sit still at desks doing worksheets, so I try to keep those to a minimum (although Lena actually LOVES to do 'school pages'). At this point in their lives, the main thing they need is exposure to a variety of ideas, materials, and experiences. Most importantly, they need to hear lots of words and live in a media-rich environment.