Once you have plenty of juice around it doesn't really matter. I order any time and they arrive whenever they arrive. If for some reason I needed a whole lot of juice by a particular date then I would order 2 weeks in advance.
The hardest part for me is NOT ordering juices. There have been times when I was ordering samples and juices every day and had stuff arriving every day.
If there is something I vape all the time and I am down to the last 10-15ml, then order more. Many of the places I order from deliver in a week, or 10 days max, as long as they didn't just have a big sale. I have 2 main flavors I vape and i try to keep them well stocked.
FreedomSmokeUSA usually ships the day you order or the next. Backwoods Brew is a one-man shop and can take longer. Tasty Vapor is so custom - that one can take longer. Ditto for Gourmet Vapor which not only lets you pick any PG/VG but also lets you create you own flavors out of up to 4 flavorings - they are rarely fast, especially if they have or just had a sale (like now - sale ended yesterday).