Congratulations on the Private Pilot Certificate! It's definitely a 'license to learn.' In fact, you will forever be learning as a pilot, no matter how many hours you fly. Enjoy the journey.
The time required for the high-performance endorsement depends on the aircraft. If you're transitioning from a 172 to a similarly equipped 182 (similar panel, for instance, and no G1000) it will likely fall in the 3-5 hour range. And, as the previous poster indicated, if you familiarize yourself with the high-performance systems and study the 182 POH in advance, you'll be well prepared. One tip on 182: make sure you're ready for the nose-heaviness, especially on landings. There's a lot more weight in front of the firewall, compared to the 172. Use enough trim and treat your landings like soft-field landings. Overall, the Cessna 182 is a great airplane! Also, keep in mind that other high-performance aircraft will still require model-specific training after you get the endorsement.
As far as glass cockpit training, this would be more useful if you plan to own or rent a specific airplane, like a new 182 with G1000. Every glass system is different and requires a specific training program.
Good luck with your flying.
Elaine Kauh, CFI