Well... I'm not using any custom content, but it still takes me a lot of time to build a screen. I rarely manage more than 3-6 screens per day. Mind you, I'm not spending all day on those 3-6 screens; I do have other things to do, too. But even on days when I have loads of spare time, I don't manage more than such a small number, because I get these kind of... mini burn-outs. I can spend a lot of time thinking about, and experimenting to see - for instance - where *exactly* an element like a tree or a plant will look best. There are even screens that, despite spending crazy amounts of time on them, I'm still not happy with.
So, I'd say, how long it will take you to build a screen depends a lot on your method of work and your aesthetic preferences and so on. I'd say my average is half an hour per screen, which explains why on most days I don't even build as many as three screens.
I also often go back to already finished screens to improve them, later.
Oh, and also: I'm by no means a real veteran, but I have played several hundred levels and I, for one, feel no dislike of the standard tilesets yet. Now, maybe that dislike is going to kick in for me later, but maybe it's also something of a matter of different aesthetic philosophies. I believe you can make something good and worthwhile from simple and/or well-known materials just as well as from elaborate and original/surprising materials. So, I'd say, if you'd like to use an 'old' tileset (or even many of them) - if you really like it and feel inspired by it: use it! :-)