Lets say I did a shop today (1/3) or even during the last week of the month (1/30 or 1/31). If the shop is reported and accepted, I would expect payment 60 days after the end of January. (Feb has 28 days most of the time.) I would be looking for payment on or before April 2. This of course means that for a shop done today I would be waiting almost 90 days for payment.
Payment dates vary by company and adherence to payment dates varies. I have companies that state 60 days from shops that I usually see payment in 2-3 weeks and then there are companies that state 90 days and you still aren't paid at 100.
I generally keep my spreadsheet by month, closing it out once everything is paid. The companies that always keep my spreadsheet open tend to be Ath, Service with Style and Service Sleuth. If there is another company keeping the month open it is time to check the pay cycle and see if follow up is needed. There are other slow paying companies that pay within their pay cycle, but I just am not working with them because there is nothing interesting in my market. At the moment I am awaiting only one company to be able to close out November 2011 and I have had a nice smattering of payments already from December 2011.