There is always a gap between pages in the buffer. The actual size of the gap is determined by the number of samples in the page just captured. For long screen times with a large number of samples this can be a couple of seconds. The gap time is the same as the "Off" period of the red LED. ("On" is the Sampling time). For the longest continuous sampling time, set a long screen time, with a lot of samples. Each screen is a continuous sample. The bigger the screen time, the bigger is the gap between them.
You have to understand the controls and what they do to get the best out of the scope. The time/div setting sets the total screen time, there are 10 divisions so 5mS/div will give you a screen time of 50mS, etc. The "Number of Samples" selector means the requested number of samples per screen, not the sample rate. The control does up to 1GS to cope with the best current scope we have which has 1GS of buffer memory. These first two settings will have an effect on the actual sample rate achieved but do not completely control it, there are the limits for maximum sample rate of the scope, buffer memory, etc. With different settings, you can have many screens (up to 1000) with fewer numbers of samples or a few screens with a large number of samples. With "Auto" triggering part of the memory is used for buffering the "Old" screen whilst capturing the "New". With "Single" triggering, the whole buffer can be used for the capture.
The sample rate selection box is a "Requested" number samples and the software will adjust settings to try to give as near as it can without going higher. The actual number of samples will thus be lower, on fast time-bases it can be considerably lower as the sample rate can reach the maximum for the particular scope.
Figures below are for a request for maximum number of samples on a 4224 (using one channel) and using auto trigger. In this case half the buffer is used to display the last capture and half is used to buffer the new.
0.05 second screen time-80MS/sec-4.0MSamples
0.1 second screen time-80MS/sec-8.0MSamples
0.2 second screen time-80MS/sec-16.0MSamples
0.5 second screen time-20MS/sec-10MSamples
1 second screen time-10MS/sec-10MSamples
2 second screen time-6.67MS/sec-13.33MSamples
5 second screen time-3.33MS/sec-16.7MSamples
10 second screen time-1.67MS/sec-16.7MSamples
With 2 channels used, you can capture:-
0.1 second screen time-80MS/sec-8.0MSamples each channel
0.2 second screen time-40MS/sec-8.0MSamples each channel
In single shot mode, more of the buffer is available for a single channel:-
0.5 second screen time-40MS/sec-20MSamples
There are also differences when using "Block" mode capture compared to "Streaming". Block mode is generally used fopr screen times less than 1second and uses the buffer memory equally split between channels in use. Streaming is used for screen times longer than 2 seconds and can stream at up to 2MS/sec on all channels for a maximum capture of 2MS per channel. The buffer memory is not used at all for this, all data is directly streamed to the PC memory. The automatic changeover between block and streaming can be over ridden in the "Tools", "Preferences", "Sampling", "Collection Time" setting.
You will often not be able to use absolutely all the buffer because the software will not allow you to select more memory than you have. In your example, your settings use 25.6 MS and the next setting higher will probably try to use twice that, 51.2MS which is impossible. (Settings go up in 1,2,5,10 ratios).
It does take a while to get used to the best ways of capturing particular waveforms. Please contact again if you need help with any other settings.