I would just go to Ilshenar and enter this command;
[facet set MinDelay x MaxDelay x where XmlSpawner
[Global will set the spawn time for every spawner on every facet where [facet is for the facet you're currently on. If you're only wanting to set Ilshenar then [facet is the one to use.
NextSpawn is a timespan value in seconds, so your command would be something like:
[global set nextspawn 30 where xmlspawner and map = ilshenar
To be honest, even if that command is 100% correct, I would prefer to use XmlFind. You can pull up all XmlSpawners and filter by Facet, then run commands against the results. Starting any command in game with the [global keyword is a scary thing in my opinion. With XmlFind you can see exactly which objects will be affected by your command before you run it.
No need to use global unless you are wanting the command to be a global thing. Your command was almost correct, but if entered, I believe it would still set them globally because of the "and" in the command, but I could be wrong. It should be or would work 100% if it were this;
[Global set nextspawn 30 where xmlspawner map = Ilshenar
Again though, could just go to the facet and use the [facet modifier instead of global.