Generally, this should be fine. The exact amount of self discharge depends on a huge number of factors, but as long as it's a good laptop battery, it usually shouldn't be empty after a month.
Expanding a bit more, self-discharge of healthy lithium-ion or lithium-polymer batteries (the two types in predominant use in laptops tends to be a reasonably slow process that is proportionate to the capacity of the battery. More specifically, it's pretty typical for most such batteries, independent of their size, to take multiple months to self-discharge if they're not connected to anything.
You can generally reduce the rate of self-discharge by:
- Completely disconnecting the battery. This doesn't strictly reduce discharge from the battery itself, but it eliminates most of the small amount of current drawn by a modern computer when it's powered off (something has to have power to check when the power button is hit).
- Storing the battery in a location that's not too hot or cold and has minimal temperature fluctuations. Based on personal experience with rechargeable batteries of this type, somewhere around 15-20 degrees Celsius (about 59-68 Fahrenheit) seems to be a good temperature, and the less it fluctuates in the short term the better.
- Making sure the battery is room temperature before using it again. Cold batteries can provide less energy than appropriately warm ones. Again, this doesn't really reduce self-discharge, but it does help ensure the system is usable when turning it on again for the first time.
Also, it's probably a good idea to go about reconditioning the battery when you use it again if it's been unused for this long. To do this, simply discharge it as far as you can (preferably until the computer just shuts off, you can do this easily (albeit not quickly) by booting into the firmware setup menu and leaving the system sitting there with the screen on until it shuts off by itself), recharging it all the way afterwards, and then repeating the process once or twice more. This isn't quite as important as it used to be, but it can help keep the battery in good condition.