If you take the Commonwealth Creek Trail that goes to Red Mountain Pass west of the summit, then it's only about 4 miles up. There is a well-worn boot path that leaves the trail just as it comes upon a flat area (about 300 yards before the pond). The boot path zigzags up the steep SW slope of Red.
Update: there is no "boulder in the middle of the trail" marking where you turn to go up to the scramble. Just a cairn on the right side once you get up to the flattened out basin - you'll know it when you see it. The "get to the lake you've gone too far" part is accurate. Also, important to note that the final summit push is 900ft of fairly steep and loose rocks/scree. Ascending is one thing, descending is another so be sure to know your limits and if you go with someone, make sure they know theirs as well. It's a sketchy scramble when with inexperienced hikers and the amount of time it takes someone with experience will triple with someone who is not accustomed. Helmet is recommended.