That depends on what frame of reference you're starting from and what time of day you're going. If you're starting the timer from wherever you are inside the park, then you have to add that time to walk out and to the station. Dinner time shouldn't be crazy packed, so you shouldn't have a long line to wait in (unless it's a really busy holiday weekend). So assuming it's a "regular" day, from the time you get to the station itself, figure waiting 5 minutes at the most for the next monorail to show up. Then, with the 3 stops before GF, figure the ride itself is about 8-10 minutes, roughly. I'd agree that 30 minutes would be a very safe bet, but barring issues, you COULD do it in under 20. Nonetheless, I'd still budget 30 to give yourself some flexibility.