Some folks start "Glendora Mountain Road" at the corner of Sierra Madre and Glendora Mountain Road in the city of Glendora. Others load bikes on their cars and trucks and park just north of the ranger station avoiding the initial uphill grind. An Adventure Pass is needed if you park in the Angeles National Forest beyond the yellow gates, parking is free within the city. I live approximately 2 miles away so I start from my garage and ride out. Still there are the real cyclists who ride from Pomona, Claremont, Upland, Covina, Monrovia or Pasadena and beyond before hitting the hill.
From Sierra Madre to the Electronic Sign less than a mile there is a short steep section that is an 8% grade. The Electronic Sign is close to the beginning of the mountain time trial of San Dimas Stage Race (SDSR). The SDSR time trial is 3.74 miles in length and ends at the new portion of the road at mile marker 10.0. San Dimas Stage Race occurs once a year in March. I like to track my progress over the SDSR portion.
Near the Electronic Sign the first Mile Marker of 13.79 is visible. The miles count down as you go up the mountain. Where the closeness of the earth to the road give way to a view to the south towards Glendora and a view to the north with Azusa Canyon around mile marker 9.49, I call this the First Saddle. This is now 4.3 miles in distance from the Electronic sign. The published grade ranges from 4 to 6.9 percent. Some GPS systems may register different readings, but your legs will know what's real and what's not. The maintenance shed is another four miles up the road and is located at mile marker 5.51. From the first saddle to the maintenance shed more of the same kinds of incline. There is one section that is 7.3 percent just before the shed. It is character building. This link will help you alot. http://www.socalvelo.com/maps/GMRgrademap600.jpg
I have learned to meet folks on the "fly". Now that the weather is better more people are on the road. For a newcomer I recommend a "low impact" time when there is less motor traffic. During normal day time hours is the best time during the week (ie normal work hours). But also late afternoon or early evening is good to ride to avoid others, however it only takes one motorcycle or one hot rod to rip your ears off. I will caution against riding late at night as mountain lions, bears and wildlife do make their home here. Saturday and Sunday there are more motorcycles on the road -- read in more danger to cyclists. Cell phone reception is ok from the start to the first saddle. Beyond the first saddle there is less probability of getting a message out if there is an emergency. Another reason to make friends on the road. Bring a spare tube or two, CO2 cartridges, hand pump and plenty of hydration.
Also use http://maps.google.com type in 700 Glendora Mountain Road, Glendora, California and then click Street View. Pretty awesome it shows you the road on a nice sunny day.