It largely depends on not only the network/internet connection to the device, but the device itself. For example, if the person is on a slower or rural internet, it may take a bit to make that first connection - and even be slow or laggy throughout the session. Processing power is also a factor. If you’re trying to remote into an old android tablet or older raspberry pi, the first interaction, let alone continued interaction, may be slow. Remember that you’re effectively not only streaming a video of the person’s screen, but also sending back actions to their computer at the same time. While Remote Support isnt exactly the most resource intensive software, it still requires resources.
If the device is a more modern device and on the local network, it should take seconds to make a solid connection. Off network or in a remote location, it shouldnt take more than 30 seconds to a minute. Personally speaking, if it took more than 30 seconds (which is a long time in the IT world), then I would have probably figured out another means of support. If it took 30 seconds for me to see the screen, how long would it take for me to see where I moved my mouse?