I think this is the best way to calculate when you will be paid (rather than counting the days).
Take the month in which you did the shop - say its February;
Then: February - skip March - skip April - Paid in May.
You have to look at your own payment history to determine WHEN in then month the payment will be made.
Mine come in between the 21st and 26th of the payment month.
Hopefully this will relieve some of the uncertainty. The only way to not feel the pain is to do as many as you can each month. After 3 months you will receive a monthly payment (even though its for 2 months before). HS Brands and Mershimer (both owned by the same person) have always been reliable BUT VERY SLOW payers. I think reliability is more important than speed, especially if you can afford to wring out the delays by shopping every month (so you get a payment every month)
Hope this helps.