That depends on what you do with the PC (e.g for gaming, game development,…). For example I have a 120 GB SSD (+ a hard disk for safety backups) in my PC and for me it’s enough, because I just use it for game development. But when you also play games like Far Cry 4/Battlefield… (alone game needs 30 GB) you should use one with over 240 GB (but then I personally would recommend you to just buy a hard disk -> cheaper and also good for storing stuff) 