What is a 'normal' resistance level for a closed switch?
What is a 'normal' resistance level in a closed switch (let's say a Push Button that works with mA currents)


It should be near Zero. Specifics will depend on the specific for your switch. The first tact switch I found on google, the Omron B3F series shows Contact resistance of 100 milliOhms (0.1 Ohms) initially. It is also only rated for 1 - 50 mA at 5 to 24VDC. It lasts a few million presses when used with 100 grams of force.


  before it needs to be considered as useless due to corrosion etc.


This is subjective. There are Two considerations.


Physical contact. If the switch has fused together, or has been stripped of enough metal that contact is not being made, it's useless.
Application sensitive.


Resistance of the contacts does not matter much in input sensing application where current flow is minimal. But it matters when being used for switching a large sensitive load.

Or when for some reason being used to switch in an analog input where the resistance is being measured. If the switch has built up more resistance than is accounted for, it will interfere with measurements. 1 to 100Ω may not matter when the measurement is looking for an order of magnitude higher (10k resistance), but if its looking for 1k resistance and your switch is now measuring 1k, that can lead to false positives.