From downtown St. Petersburg, it's about a 20- to 30-minute drive - just head west on Central Avenue and continue for 9 miles, through historic Kenwood and the Grand Central District (home to many of St. Petersburg's best gay bars, including Georgie's Alibi and Queen's Head, crossing the Causeway to Treasure Island, and then turning south onto Gulf Boulevard (Hwy. 699). Here are the directions on Google Maps. Here's a complete guide to St. Petersburg Gay Nightlife.
Once you reach Gulf Boulevard, it's about a 1-mile drive to the 7-Eleven at the point with Hwy. 699 cuts east, becomes Blind Pass Road, and leads to St. Pete Beach. When you reach this 7-Eleven, bear right and continue south about a mile along Gulf Boulevard until you reach the metered parking lot just north of the Mansions by the Sea condo tower (pictured here). Here's a look at the drive from the 7-Eleven to the parking lot on Google maps.
If you're come from Bradenton or points south, take I-275 north over the Sunshine Skyway Bridge, exit west at Hwy. 682 (or 54th Avenue), which is exit 17. Follow signs for St. Pete Beach, and once on St. Pete Beach (another barrier island), continue north on Gulf Boulevard (Hwy. 699), until you cross onto Treasure Island and see the 7-Eleven mentioned above. From Bradenton, the drive is about 45 minutes. Here's a view of the drive on Google maps.