How often should I empty the recycle bin on a SSD drive? Should I let it completely fill up the hard drive first? I am on windows 7 with TRIM enabled drive.
Moving files to the Recycle Bin is only a directory change. As the files are not physically moved on the disk, no additional writing is done on the SSD, so no harm is done by keeping the Recycle Bin enabled on the SSD.
Disabling the Recycle Bin only means that files are deleted and trimmed immediately, but with no real benefits compared with their being deleted later on when the Recycle Bin needs the space.
Filling up the hard disk with Recycle Bin files might cause some products to fail with insufficient disk-space messages, so this is also not recommended.
On the other hand, emptying the Recycle Bin too often might mean deleting a file that you might need.
My advice is therefore to leave the Recycle Bin alone and let Windows manage it.
By preference you do not want to have wasted space just sitting there on an SSD, I'd empty it as often as you can.
Emptying the trash can will free up space on the drive that the wear levelling logic on the controller can use for other more deserving purposes, it will also allow the TRIM command to work in advance and clear that area of the drive for future writing before you actually need it.
At the very least an SSD will work better and for longer if it is not completely full.