Ah, so not too long then. I think it probably is a combination of poor conditions during transport and at the pet store.
As for your infrequent water changes; it's okay, we've all done it. The important thing is to learn from your mistakes, and try to set up specific times and days of the week to set aside for tank maintenance.
For the other products that i mentioned, i usually pick mine up at a local pet store, or online. I got my AQ salt from amazon, for around £5 (it comes in a sort of "juice carton"), and I've just bought a gravel pump for £7. (smaller ones are cheaper, i just got a large) The bacterial control i picked up at the pet store for about £6, and i got the King British one. I'd also recommend the brand Prime, and apparently Tetra are rather good too, however i haven't had any experience with any of their products so i can't give you a personal opinion there.
Also, some useful information; I'm not entirely sure about this, but I've read that mollies can get quite stressed in smaller tanks. They like lots of hiding places, but apparently they like to be able to roam a little, of which they can't do in smaller tanks. Not having this opportunity can actually lead to health issues, and premature death!
I have a 17L tank, and i used to have three mollies. All three of them eventually got freak illnesses and died. Just bought a 60L tank in case, so i'd recommend getting a decent sized tank if you haven't already. (Just wanted to share in case this is true, bc this was definitely not something i was told when i bought the fish)
