My first thoughts about this week's question are that we only really think about inner strength in times of hardship - that is inner strength is something we need when we have to overcome adversity in our lives. It is a reserve we draw on when needed to get us through 'tough times'.
When everything is OK, we probably feel that we don't need inner strength, life just sort of happens, we can relax and go with the flow.
This, I think, is the mainstream interpretation - life is tough and so we need to develop inner strength (resilience skills maybe being a softer version of this) to help us through the tough times - ways of coping with anxiety, unhelpful others, tiredness, too much to do and so on.
This way of thinking about inner strength was becoming very fashionable towards the end of my full-time teaching career in 2018, with classes in mindfulness and well-being days for staff and students alike - and yes, meditation, yoga, all of that kind of stuff was there to help people 'cope' and to develop 'inner strength'.
However, Id already come to the conclusion that the education system was simply fundamentally unsatisfactory long before the institutionalization of resilience, and so cutting it out from my life was the only effective solution for me - there's no point trying to cope with a broken system, no point using one's inner strength to manage and struggle within - eventually, if you're powerless within such a system, if you have no identity with its logic, you'll be overwhelmed, so escape is the only way.
So maybe my first point is this: you need to be very careful what you are using your inner strength for - make sure its pointing in the right direction, rather than just a coping strategy for the long term!
Inner Strength - should be always on
Personally, following Bikhu Bodhi's interpretation of the Noble Eight Fold Path, I think that inner strength is something you need to draw on constantly, maybe even more so when you're experiencing the 'good life' when 'everything is going OK'.
I remember when I was proper into Zen, over a decade ago now. After a few months of a lot of meditation I made a practice-observation during a Monday night meditation session, or at least after the actual meditation - it went something like 'I've noticed since meditating a lot that I feel uncomfortable and dissatisfied on Saturday mornings, which is traditionally the 'best time' of the weekend'.
The response from the teacher was simply 'go deeper'.
So what inner strength means to me is a sense of constant vigilance, constant right effort, and right every other aspect of the Noble Eightfold Path. If done in unison and constantly, it's actually easier to stick to than switching in and out of!