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.