As a yard stick to go by.
My long range(ish) quad is my ImmersionRC Xugong 2Pro. I’m using the DJI E310 propulsion system (9x45 props, 2213 980kv motors, 15Amp sinusoidal ESC’s), ImmersionRC EZUHF, NazaV2, ImmersionRC 600mW vTx, 5200mAh 3s 10c Multistar. It’s also carrying a 2axis gimbal and GoPro H4B.
I’ve only been out to 2miles with it, this is due to battery capacity/voltage (a 4s would have less current draw). Both the control link and video were solid at the 2mile point, the EZUHF was still showing a 100% link quality (no dropped packets) and RSSI was in the low 30’s (the EZOSD works backward so that the lower the number the stronger the signal).
Two other radio systems to consider are the recently realeased ImmersionRC Ghost which is a LoRa system on 2.4GHz, the receivers are tiny, and the Skydroid T10/T12 which also provides digital video.
I have the Skydroid system fitted on one of my test quads however I did a static test with the air unit mounted on a six foot pole at my flying site and then drove out with the base unit, see map below.
During the drive out I did have video dropouts but this while in very low congested areas, the river Wharfe flowing through Otley is 60m ASL. However at the destination point, which is about 220m ASL (the flying site is approximately the same altitude) both the video and the control link were solid. It’s worth noting that even though the video is touted as being 720p the frame rate is not very high and there is some latency. For steady cruising purposes this is not an issue but I wouldn’t use it for acro/aerobatic flights. Skydroid claim that the T10 system, the one I have has a video range of 10miles and a control range of 20miles. I have no reason to doubt this and I have seen some impressive claims from other users in forums and some YT vids. It is designed to be used with Pixhawk but I have seen some using it with other F4 and F7 controllers running Ardupilot.
With the price, power requirements, and size of LoRa devices dropping I can see other manufacturers taking up the technology. If you search LoRa on Amazon there are plenty of cheap modules to experiment with on all of the ISM/Licence Exempt bands.