It can be pretty time consuming to learn what you like and dislike without even getting in to the different aspects of mixing and methods. Starting out you will likely end up mixing quite a bit to find your preferred taste, including doing research to find flavor recipes and other things you can get quite caught up in everything initially. Personally I would allot around 1hr per juice recipe you intend to produce when starting out, this time will drastically decrease as you do it more and more.
Now having said that, I have moved far past the initial stages and I have four main flavors that I have been using for around a year now. Throughout that first year I learned a lot of tips to save time and one of the biggest tips I could ever pass on to an aspiring DIY'er is to measure EVERYTHING by weight so invest in a quality scale which can handle at least 300g and reads to the hundreds place(that's two digits beyond the decimal, ex 0.01). Really I cannot stress enough how much of a HUGE time saver mixing by weight is as all I need to do is put my large batch bottle on the scale and simply hit "tare" between every ingredient I add. No time spent cleaning syringes or pipettes, nothing gets dirty at all since I am able to pour straight from my flavoring bottles. There are numerous online Juice recipe calculators which will tell you the specific weight of the ingredients you wish to add.
Now that I have my recipes saved and am accustomed to mixing by weight I can mix a full 250ml batch in under five minutes from start to finish, so once you're a little seasoned in the methods of DIY you will need less and less time.
How much time does it take you to leave your home, drive or walk to the vape shop, debate over a flavor and then leave to drive back home? I'm fairly sure for myself that DIY has saved me a ton of time in travel alone.