How strong does the feeling of Loving-kindness / Metta have to be to progress?
It doesn’t have to be strong – it just needs to be continuous.
Make sure you 6R whenever your mind wanders. So, whenever you forgot you were actually meditating.
Every time you use the 6R’s, the feeling of Metta will be able to grow, as the mind will continue to calm down.
Whether strong or weak, just watch mindfully for the moment the mind starts to wander.
The longer you can stay on your object, the stronger the feeling usually becomes.
Don’t push, just 6R when needed.