Here is how to get this trophy (and the 60 second one):
When you are close to the ground or walls, you will see the red lines indicating a grind. You must keep these going for 15 and 60 seconds in one continuous grind. It has to all be in one go, and cannot be interrupted. You do not need to, nor should you activate boost during this period.
Here are the essential tips for these trophies.
1) Use score attack mode, as you cannot do the 60 second one in Race or Time Trial, since you will be disqualified for missing too many rings.
2) Listen for the announcer lady to tell if you have broken the streak. Once you start the grind, she should not say anything until you finish it. If you hear "Boost Full" or "Awesome" or something to that effect, that means you broke the grind and will start over from 0 seconds the next time.
3) Here is the one that tripped me up. You must successfully complete the grind, i.e. come out of it and end it, to get the trophy. If you do it for over the amount of time required, but then crash before coming out of it, you will not get the trophy. I thought this was bugged because I was grinding forever and never getting the trophy, but it was because I was just going as long as I could until I inevitably crashed. Only when I accidentally broke the streak after 60 seconds did the trophy pop. This can be tricky as you may not be sure if you have gone on long enough before breaking it, but risk crashing if not breaking it.
4) Location 1 for the 60 second one - The snowy stage, Start by going down the middle, then sharply over to the right into the canyon to the right of the first tunnel/cave. Then just follow this smoothly down the mountian, going to the right once approaching and going under the bridge.
5) Location 2 for the 60 seond one - The 4th area with the jungle canopy trees. Just stay on the right side of the course all the way down. This is the most direct straight shot, but can be a little more tricky than the snow stage as the canopy floor can be very bumpy instead of smooth like the snow area.
Hope this helps.