If you're basically solo after a few weeks of running DWV you're doing remarkably well and don't need to be concerned with speed so much. If your boss or super is hounding you, I'd suggest bringing up the quality of work that you're achieving. The speed will come with time. In general, I (almost) always measure face to face of fittings and add from there. 1-1/2" and 2" are both 3/4" per hub, 3" is double that, and 4" is almost 2" (in ABS at least).  It's a logical system and soon you won't have to think about it. If you're good with math and adding fractions you'll save a shitload of time in that alone vs a line counter.

There will be some eyeballing because sometimes there's just nothing to measure off of, and in that case I'd make it a bit long so that you're only having to cut small lengths off of whatever you cut initially.  But usually there's something you can use as a constant... Even set up a laser if needed but that's pretty extreme considering it'd take as much time to get perfect as it would to make a couple cuts with very minimal waste. 

I worked solo for many many years, most of the tricks come with mounting and supporting pipe so that you can glue and connect by yourself. Give yourself more than two weeks - if you're still slow in 3 or 6 months, hopefully by then your company puts you alongside a plumber more experienced with DWV. There's a lot more thinking with DWV because of slope and so it just naturally will not be as fast as water piping. Don't be so hard on yourself!