There is no simple answer for that question, it mainly depends how organized you are, what kind and what size of software you gonna keep on it.
For example on my stationary PC, my system drive (Samsung EVO 840 120GB SSD) has only WIN7 and some vary basic software + 1-2 games that i frequently play in multiplayer so i want them to be as fast as possible. Other non multiplayer games are just stored at secondary drive (WD Blue 1TB) since i really don't care about few second faster loading times, most recently used but not crucial files are also stored there. At the 3rd drive (RMA'ed, refurbished, crappy Seagate 1TB) due to its lowered reliability and performance i store there only game installers and movies so the files i seriously don't care about or are used once or few times a year. I have also few drives in a closet not worth mentioning either dead or retired.
Anyways to make this long story short 128GB is quite sufficient for my needs but you are gonna be really conservative about your installation planing.
On the other hand i also got laptop for my work and it also has 120GB SSD primary drive, OS+VS2012+MSSQL server 2012+Clarion+Eclipse and few SDK's, Oracle Client, SVN repositories and few database backups restored... well it gets filed up in no time. As much as I'm keen in planing my storage requirements this laptop is good example of how hard the decisions has to be.
So yeah its about this... if you are good in planing your storage requirements 120GB SSD will be good, but if you tend to install all the crap on OS drive then even 500GB can be sucked out in no time
And no oh no with only SSD build.... i would rather buy 120GB SSD even some cheap model and have some 2TB Hitachi or WD or drive instead of 240GB or even larger SSD drive, thats really suicide mission imo.