shopfactory is payware, it is very very complete (imho) but it has a price.
If you only want a small shop (as in options I mean) then things like prestoshop and so would be fine… And then building it in sc is also something you can do…
How much time it takes depends on the payment methods you want to accept, the rest is not that hard in scriptcase. It depends if you want to have a simple shop or one with lots
of options, and customer programs, many payment methods etc…
A basic shop with paypal could be up and running in less then a week (provided you already have the database with products).
A bit more advanced shop with an entry for customers (buying) and emloyees (doing the orders and adding to supplies) is clearly more time. You first have to determine what you actually need in your shop. A super advanced shop (if you insist in building it) will probably take you a few months. Be aware that get the payments methods realised also takes lots of time, it needs approvals from the banks, credit card companies, paypal, etc… with sending paper/documents to and fro. There are several paypent providers that handle multiple ways of paying, for a small fee per transaction you win a lot of time using their php code for handling the finances. In fact there are easily more then 40 different online payment methods and building them all takes time… Hence a standard shop or sticking with paypal and at most a few others (the major ones).
Be aware that for a shop you also need certification and verification and ssl certificates and so on, to prove that the shop is legit, that the money is safe, the communication is secure, etc… That takes its time too (sadly enough sometimes several weeks).