What is the impact of leaving a laptop in "sleep" mode (while on battery power)?

It depends on your system.

Most people I know using modern laptops use sleep as the most common method of leaving the machine. For laptops I have had, I always let it sleep overnight or even over the weekend. From a max battery, it may last several days.

However, I know others who rarely use the laptop so they hibernate instead of sleep. For them, the laptop is used maybe once every couple of weeks.

I leave laptop regularly at sleep overnight. Consumption is low enough to last for more than a week on my model.

While consumption does depend on motherboard and other components, you should be able to get few days of sleep without problems.

Throwing laptop to sleep is very easy to get used to and I don't even recall when last actual reset occurred on my computer.

I asked almost the same question here.

As for the shutting down completely thing - Windows needs to be rebooted once in a while to complete certain scheduled tasks. I would say to at least attempt to reboot once per day, but other than that, you should just sleep it for a quick launch back into the computer.

You can easily leave the thing overnight on sleep mode and it won't drain much battery at all.

Regarding how much battery it would consume varies based on processor, motherboard, and additional onboard components. As far as a "recommended time", it is a personal decision and up to you and your environment. I have my sleep disabled. I occasionally force it to sleep when I want to for extended periods of time (30mins or more), but it's usually always on except for the trip to/from work, so i just shut down. vista performs better that way anyway.

I would not put a laptop to sleep overnight. If you really want to keep it "running", look for a hibernate option instead. But the best thing to do is save your work and shutdown. Laptops really aren't intended to be left on all the time.