There is no best time to invest, the best time for you is when you start.
There is actually a best time for one to start their bitcoin investment, that best time was yesterday. This is Just to let you know that there is actually a best time for investors to start their bitcoin investment journey.
You only have to invest with money that you will not need in the next 2 years.
Why the choice of 2 years? What happens after the 2 years has passed and you have need the money? for me Investors should invest only the money they know they are not going to need as long as their accumulation and holding period is going to last. If you intend holding your bitcoin for one circle which is 4 years, invest money you won't need for that number of years, and so on. 2 years is not up to one full circle of bitcoin. I don't know the reason behind your choice of 2 years, but doing it this way will give investors more flexibility.