Podcasts take up as much space as songs do. It depends on how long the podcast is. If it's an hour long or something, it may be 30 MB or so. You can find out by going to the podcasts section on iTunes and find out the total file size or find the "Get Info" for each individual episode and find out there as well.
I subscribe to quite a few podcasts, right now I have 47 podcasts I have yet to listen to. That adds up to 1.85Gbs. Video podcasts do take alot more room though. Most people only keep unplayed podcasts so iTunes will automatically delete old episodes once you've played them all the way through.
It also depends on the bit-rate at which the podcasts are encoded. Some podcasts encode at 32kbps or 64kbps so as to take up less space. The audio quality may not be as good, but if it's a podcast with nothing but speech on it, it's really not going to matter.
It should also be noted here that even though Podcasts do take memory on your iPod, iTunes can be set up to remove old podcasts from both iTunes and the iPod. Both iTunes and the iPod have some options that may be set to limit how many episodes are stored and/or transfered to the iPod.