At some point we had 10m intercity intervals between Rotterdam/utrecht and Utrecht/Amsterdam in NL.

Seems like it's 4 per hour on Rotterdam/Utrecht, seems similar to Geneva/Lausanne with 6 per hour.

In any case, I think commuters are fine with every 15 min, as long as there's enough seats. (for long distance like trains, my feeling is that frequency below 15min doesn't have a lot of impact, unlike shorter distance public transport like tram/bus/subway)