For the Lake Poway side, are there “opening” hours or can you hike as early as you want?

I didn't see any rules posted,but I parked on Hwy 67 near the Cal Fire station and walked up the trail from there. I did not enter from Lake Poway. We combined the fire road and the Fry-Koegel Trail for an enjoyable 5.6 mile loop. The Lake Poway Park web site says " Lake Poway's gates open at 6:00 AM daily and are closed and locked at sunset. "

You will have to park outside the park if you want to hike before they open around 6am.

6am to roughly sunset for the lake poway trail. On weekends and holidays there is a $10 fee to enter the park if you are not a poway resident.