According to the strategy guide, you need to travel thousands of light years into space to increase the chance of encountering legendaries - at least 2,500 light years, and at 4,000 light years the rarity bonuses stop.
There are 4 types of warp hole. The further you go the more likely you are to encounter the rare ones
- Warp holes will almost definitely never have legendary Pokémon
- Warp holes with a light orange ring will rarely have legendary Pokémon
- Warp holes with a light purple ring outside of a yellow ring will sometimes have legendary Pokémon
- Warp holes with double purple rings will frequently have legendary Pokémon
The tips to get further is to avoid the electrified (blue) spheres and collect as many orange spheres as you can.