Using the glass plate method (I used 2 pieces of 1/2 Laminated glass) it would often take an hour or two at the low heat. I never placed weights on the top plate for fear of crinkling the lp. I feel it is better to let the lp heat up very slowly and cool down over night before removing the weight. One note of caution for any process, some vinyl is of poor composition and might self destruct. I offered to straighten a warped early, off lable, Lou Reed and John Cale live Drella lp. It completely melted out in areas, blurring the grooves. It was clearly substandard vinyl. The fellow was very understanding. This might bring up issues of liability....