How realistic is bricking? As in, how often would this occur, do u think, on average?
Going solely on what I've read online.... next to never.
The only time I've heard of the PSP bricking is when the battery died mid-flash (which is why its recommended that you plug in your power adapter), and by someone who had literally flashed and reflashed the PSP quite a few times... dozens, if not 100 times.
It really shouldn't be much of a concern. Plug yourself in and make sure your battery is charged. Beyond that, just don't keep upgrading and downgrading, and you will be fine. Plus, nowadays, there really is no reason to upgrade/downgrade anymore like previously... so once you're at 1.5, you shouldn't ever have to upgrade/downgrade back to 1.5 again.