How long should a punishment be for a pre schooler?

One minute for ever year of her age - then go talk to her.

Then you need to secure your stuff so she can't get at it. At this age they have little impulse control so don't set them up for failure.

The rule of thumbfor time-outs is one minute for every year of age, but if she's been told often not to get into stuff and make a mess in the bathroom, it won't hurt her to stay in her room a little longer. I probably would have made her help me clean up the mess instead of sending her to her room because I reserve that kind of isolation for behaviors like tantrums or really serious talking back that make getting sent away from other people for a while seem logical.

This is normal curiosity. If you set her there for more than 3 minutes for every year of her age you have set yourself up for failure. She will not remember why she is there only that you are being mean, and you are.

It honestly depends on the child. My 3 yo needed about 7 min of time out. at 4 1/2 she needs closer to 10. This is so that the reason for the punishment sticks. I would have also made her help clean up the mess and taken a privilege away from her....like her bedtime story or watching her favorite show.

I always did the time for the age of the child. We also had a chair that they had to sit in. When the punishment was over we would talk about it - "do you know why we had to sit in the chair? how did you feel doing things that you were not supposed to do? Do you have anything you would like to say to me?" and 99% of the time my kids would NEVER do it again. Spanking was not ever an option..it was more of time out and a good talk.

Lock your stuff up. Get cabinet locks.

I have the same problem w/ my 2 & 4 y/o's. We put hook and eye latches on the doors of the rooms that they weren't supposed to go into, but they figured out how to open them. Even the higher cabinets in the kitchen, they have both figured out ways to get into them, and they always make a mess! The fridge is another one they are constantly getting into. ARGH!! It's maddening!! But I've just recently started putting them in their rooms for a few mins. I agree that the amount of time is diff. for each child though, and explain why they were in time out, & then make them help me clean the mess up. They are still getting into things, but I just started doing this type punishment, so only time will tell.

In our house, it's usually the stairs that are used as the time-out spot. There's nothing to see or do, unlike the bedroom, which has toys in it.

As an alternative, since locking that cabinet/using a different one isn't realistic, you could try to address the issue of sneaking. I've found it helps to focus on what I DO want in a situation (I didn't want my child sneaking into my dresser; I wanted it either not to happen at all OR to be there as "damage control," instead of coming upon a situation after the fact. So I pinpointed that I wanted to be involved, to supervise & to prevent certain things.)
So instead of setting her up for failure, then figuring out how to punish, you could try to connect to her purpose in the behavior (what is the "payoff" for this, what is interesting or fun to her, what is she curious about?) Recognize that it's exciting/fascinating & acknowledge that (you're identifying her "positive intent" which connects you two), and let her know "When you want to see those things, come & tell me."
Then follow through.When she asks, drop everything & go.