There isn't to my knowledge an objective definition of early and later reflections. But normally we're talking about early reflections till 10-12 ms area and late after that.
The graph is from Peter D'Antonio's book but I don't think it says from what specific room. Below is also the before impulse.
View attachment 76250
The RFZ or ISD (initial signal delay) gap of that graph is around 18 ms. Where you see the sudden rise in level, is where you have the termination of the ISD gap. How long should the ISD gap be? In a control room and with a LEDE/RFZ design the ISD gap needs to be longer than ITD (intial time delay) of the recorded room. It also needs to fall in the Haas zone. The idea is to hear no control room reflections until after you have significantly heard the reflections within the recording.
Take note the LEDE/RFZ design is something else than a "reflection free zone" and was a development of the LEDE design.
In a listening room, the ISD gap will depend on the room geometry and placement. But ideally, a ISD gap of 20-25 ms would be ideal because it enables one to hear much of the recorded signal, it's within the Haas zone and similar to the best concert halls in the world. In many small rooms, such a long ISD gap isn't possible though and has to be shortened. You basically do what you can do from the type of room you have. But a shorter ISD gap will not be as spacious and lively of one with a longer one besides not giving the ability to hear as much of the recording room when that's appropriate (many modern recordings are done in tiny rooms).
There's a lot that could be said about this. For example the termination of the ISD gap is very important. The level will decide how much liveliness of space you achieve and how well you can cover earlier arriving audible reflections. While treatment will attenuate specular reflections, it's common to still have some minor ones that's audible. A strong termination of the ISD gap tricks the brain to overlook these besides giving a boost of energy/liveliness which is very pleasing. The purpose of the ISD termination is also to remove the localization cues of the later arriving energies and to reinforce the localization cues of the direct energy. There's quite a bit going on here and how it relates to each other.