The answer to this question has a few variables that makes it difficult to give a direct answer. The one consistent answer we can give is that you can generally "begin using" the melter within 25 - 35 minutes of firing it up from a cold start.
The two biggest variables to consider are outside temperature and wind. The colder or windier it is, the longer it will take to melt rubber. The size of the melter and amount of rubber will also be factors. Just make sure you maintain the melter temperature at the recommended temperature (usually 375ºF - 400ºF) and always keep a new block of rubber on the already melted rubber to maintain best performance. Exceeding the rubbers melting temperature can burn the rubber and alter its properties. Double check your rubber instructions for proper operating temperatures.
Without going into a lot of detail about the process, it generally takes between 1 - 2.5 hours depending on melter size and volume of rubber to fully melt the blocks from a cold start. This is true on pretty much any direct fire melter regardless of brand and design. The thing to note though, is that you can most certainly use the melter before the blocks of rubber have fully melted.