It varies model to model, so no real way to answer that question.
Generally, when a SSD gets nearly full it will start to slow down. Another general rule is that the larger the capacity of the drive the better the performance when it's nearly full. So a 120GB drive that is 80% full will work better than a 60GB drive that is 80% full.
Run a benchmark right after you install Windows and your motherboard drivers. Keep it as a baseline and run another one after you've installed the rest of your programs. You can then compare the 2 benchmarks and see what kind of performance hit (if any) you've taken.
You've already purchased your SSD so it's too late, but there are benchmarks with the new OCZ Vertex 4 that just came out last week with the drive at 100% capacity with hardly no loss of performance. See Post #4 of this link: http://www.ocztechnologyforum.com/forum/showthread.php?100402-Vertex-4-performance-and-analysis