It depends of what precursors you are using. Finally what property you want to achieve. If you elaborate what you did for synthesis and what you want to achieve, I may give suggestion.

I've used ammonium molybdate for the impregnation and nitrates forma che co-precipitate support; my aim  is to achieve an high superficial area. I've previously tried a temperature of 400C

Sometimes a knowledge of Ce/Mo oxide phase diagram can help. However, I will suggest that you try calcining at higher temperatures and run the XRD to confirm the phases at every 100 degree. When you've gotten the right phase, then you use smaller intervals of 10 degrees to get the right temperature. 

You could try a hydrothermal treatment.

Quite often hydrothermal conditions lead to crystalline phases, whereas the "normal" condition formed an amorphous phase.

So, take your synthesis solution and keep it in an autoclave at 150C or 200C for some time (e.g. 24 hours). Then check the crystallinity and see if anything formed. Depending on the results, you may then decide to use a different (higher) temperature or to run the experiment for a longer time.

Thank you all for your useful answers, i'll try my best to test your suggestions

I had worked with Molybdenumtrioxide and Manganese dioxide.As per my experience,

you may look at the phase diagram of the reactants which would definitely give information.In its absence, I would recommend a temperature between 300 to 400c for calcination for about 3 hours.I had observed that molybdenum oxide reacts at higher rates compared to other oxides like ZnO ,Al2O3 etc.As suggested earlier by others,you may check intermittently whether the required structure of the compound is obtained or not.

all above comments are very useful. however, I would recommend you to apply TGA/DSC method to see any (degradation, decomposition, phase transformation, MoOx sublimation, etc) process. this method can give some idea to find your working temperature range. another important parameter is the heating rate, because high superficial area is may relate to gas generation (NH3, H2O, etc.) and melting/decomposition process kinetics. its not only up to calcination T and duration at high temperature. good luck