12mm tiles on ceiling !

I tiled a windowsill in a bathroom with 10- 12mm porcelain tiles. The walls and floor are OK of course but I'm nervous about the top tiles attached in case they fall later on. I had to use tiles with existing dried adhesive on the back (which makes them heavier) coz they'd run out and the other tiler had run off - so I'm finishing his crappy job. I decided to glue them on with fast grab solvented adhesive as they're thick enough already and hoped this would be strong enough. They're sticking onto plaster board only which I PVA'd first and propped up now with wood until the glue dries. Any advice about the safety or method would be greatly appreciated. Cheers