Historically speaking, Bitcoin's dominance has only started decreasing when Bitcoin broke its previous all-time high.

This means that the best time to start investing in altcoins is when Bitcoin breaks its previous all-time high.

That's when retail FOMO begins to kick in.

I remain fully invested in Bitcoin, with a small amount of Ethereum, and only a very few select altcoins (in minor amounts) that I believe could outperform Bitcoin.