It is possible for RAM, CPU and motherboard to go wrong so a failure does not pinpoint the failing item. My guess is it happens more frequently with RAM but that guess is not based on any hard evidence.
It can sometimes be worth simply reseating the RAM in its motherboard slots in case it has worked slightly loose.
It is possible for each RAM stick to test out fine individually but you still get failures when you have multiple sticks plugged in due to overloading the motherboard memory controller. Carefully check in your motherboard manual the maximum RAM speeds your motherboard+CPU combination can support and remain stable - it is often lower than the rated speed of the RAM sticks, and can vary according to the number of sticks you have plugged in.
Anything other than 0 failures means the system will be unreliable. In terms of how long to run the test the general answer is at least for a complete pass, and ideally for longer (e.g. overnight) as long as you are getting 0 errors. No point in continuing a test once you start getting errors reported other than perhaps seeing if it points to a particular RAM sticks/slot.