No, this is not true. One of the main advantages of stock photography is that your royalties will keep on coming over a long time interval. It is nevertheless true that if you keep uploading, the sales increase much faster. Their amount can double or triple over the next month, if you upload constantly. After uploading a large batch of images a significant number of sales may appear after one month or sooner. Once you stop uploading, the sales do not grow as fast as before, still they will not collapse.
These are conclusions based on statistics, your mileage may vary. Uploading good photos constantly and constantly improving quality as well as aggressive selectivity of your own portfolio are the most important "secrets" of success in stock photography.