Rendertime is easily estimated... Just divide the size of your image to render by x and render it at the settings you want. Then multiply the true time by x^2 and you have an almost 100% good estimate. So let's say you would want a render of 800 x 600. We make x=10... So render it at 80 x 60. If it takes 2 minutes then the real time will be 2*10^2=200 minutes. If you think 80 x 60 is already too long to wait then make x=20 so the pictogram you're rendering is 40 x 30...