Hi C.,
2.5 to 3 hours is the normal length of the nursery school day; if you live an an area with 1/2 day kindergarten, 2.5 hours is the school day so it's really not beneficial to send the kids for a longer preschool day.
That said, I would not choose a one day a week program. Your daughter is very young, and especially if she doesn't do well with separation, just going one day a week won't get her into the routine of preschool. Kids that age typically don't settle in when they have to relearn the separation, the routine, etc each week - or longer if she misses a week due to a holiday falling on her school day, illness, snow day, etc.
When my oldest was preschool age, I opted not to send her at 3 years (she was already in a daycare setting with other kids her age fulltime), and instead sent her to a half day program at 4 years old - they bus picked her up and dropped her off at daycare. This school let you choose full, half or 3/4 day, and 2, 3 or 5 days a week. Since she'd be going to 1/2 day K, I chose a half day program but opted to send her all 5 days, just like what K would be like
When my 2nd was preschool age, I was primarily a SAHM, only worked a bit and I sent him at 3 years old ... a year earlier than my daughter, because he didn't have the exposure to other kids that my daughter did since he was not in daycare and he wasn't as advanced in his academic skills as she was. The school we chose had a 2 day a week program for 3 hours for the 3 year olds, and the next year he went to their 4 year old program which was 3 mornings a week.
Good luck in making your choice!