The NetFlow table shows 100, 500 or 1000 flows depending on the setting under Query Options.  If you want to see the flows after that, it can only be done with a custom time range.  This is assuming the data is sorted by date/time order, which is usually the case.  Using the custom time range is cumbersome, error prone and not easily doable, given that there will be lots of flows with the same time up to the second.

One possible solution is to use pagination NETOBSERV-291 which is preferred. To simplify this, you can only get the next set of data (or previous data if applicable) when the data is sorted by date/time order.