Actually most fruit flavors require very little steeping time IMO, although ANY juice will change over time. I always run the bottle under hot tap water and then let it sit in the water for 10 minutes. Allow it to sit and return to room temp, shake it vigorously several times and it is ready to vape.
As for LorAnn flavoring, many of them are pretty good. Raspberry, Strawberry, Cherry, Bavarian Cream, Keoke Coffee and English Toffee are all pretty good. The Watermelon is only so-so. I have also used TFA and Flavour Art which are very good. I just ordered a bunch of flavorings from Mt. Baker Vapor and am looking forward to trying those. Taste is very much a personal preference and everyone's opinions on that will differ. Personally I can't imagine mixing whipped cream and watermelon
Good luck with your DIY experimentation! Trial & error and patience are required but it's worth it when you mix that first perfect batch.
Edit: And yes, use the advice above and start with one-flavor batches. Find the right percentage of flavor you like. Then, if you mix a great bottle of strawberry and a great bottle of cheesecake you can simply add them together for a delicious vape.