IMO 30cm is fins thats what I have and no problem sitting there but a bit deeper is nice if you have the space , You will need support under it no matter how thick the counter and there are really nice invisible steel ones that are great.I find legs on an overhang really limits seating I like a simple eased edge on counters unless you have a very formal/ traditional kitchen which BTW does not usually include shaker cabinets . I do hope your seating spot is the same height as the rest of the counters and not raised .

A countertop seated-side depth is usually recommended to be a minimum of 38 cm (15 inches) for comfort. You can go less but may not have a pleasant experience with it due to knees bumping up against the wall. I have a 20 cm (8 inch) overhang that is completely unusable as a seating area due to its being too shallow, and I plan to increase it substantially during renovation.

As for slab thickness, a 2 cm slab will definitely need additional support for an overhang as deep as you are planning, and really a 3 cm should have support as well for anything beyond the 8" to 10" range (20 to 25 cm). If you want to reconsider and extend the overhang closer to the NKBA-recommended 38 cm, then absolutely plan on an additional support system. Delicate stones may need support for shallower overhangs than the specifications stated above. Fabricators I've talked to are amazingly relaxed about these support recommendations, but it's a tragedy waiting to happen if a stone countertop were to fail and fall at the wrong time. You can find a fair amount of documentation on these recommendations from organizations that provide information about natural stone fabrication and use.