Depends on the weight you expect the cabinet to support.
The more unsupported distance you have the less weight it can support. You may need to think about how it may be used in the future.
I've "built" 3 x Ikea cabinets @ 600mm wide, by 600mm deep x 400mm high. I want to make them look like they are floating and so don't want kickboards.
The only info I can find about how far back from the front and one side I can go with a plinth, is the standard 50mm to 70mm.
That's not far enough for my plan: I don't want to see the plinth.
I can build a solid plinth all the way to the back and the other side: can I go 150mm back? or 100mm?
Depends on the weight you expect the cabinet to support.
The more unsupported distance you have the less weight it can support. You may need to think about how it may be used in the future.
Yes you can go 100mm or 150mm back from the front edge, this won't be a problem.
Anything more and you are asking for trouble because IKEA cabinets have that flimsy back on them and only fix by those weak brackets.
If it was regular solid backed cabinets you could move the plinth back a lot further, as the solid backing would happily hold the cabinets in place with some additional screws added to the cabinet sides.
Make the plinth black so it disappears, make sure to attach the cabinets to the wall securely, especially at the top of them.
Mieux vaut prévenir que guérir