The last time I made an order, the stickers were shipped the following day.
However, as you acknowledged they are backed up from the sale. And it's a first come, first served basis.
Another factor in delay is if you're ordering an older sticker set they don't normally keep in stock, they have to have it printed first which may delay your order being sent out.
However never fear, your order will arrive. Just a matter of when!
If after 3 or 4 weeks you have any concerns, you should contact them. They have really good customer service and will generally get back to you ASAP.