For non-CBR rate control, it isn't possible to predict how long the buffer lasts, because the space required depends on the complexity of the scene. Increase the Maximum Memory setting. If you want 600 seconds and get 30 seconds with the current maximum memory, multiply the current maximum memory size by 20 so there is space for 20 times 30 seconds = 600 seconds. This will get quite huge, if you record with high quality/high resolution settings.