The best time to plan is dependent on how stable/unstable the environment is.
That is, without spending 10x to 100x the time up front to get it right the first time. But if you're not building space ships or nuclear reactors, it's so much faster and better to just do it and figure out things along the way. So much time spent planning and guessing about the future is time wasted and that's why early stage startups can do something in a week that would take an old enterprise 5 years.
"no plan survives first contact with the enemy"
And that's the source of agile and why too much planning is just wasting time for management to have something to do. No I don't know exactly what I'm going to do or when it'll be done, and if you leave it like that I'll get it done faster.