Hi and welcome to HCM,thank you for your query.
It depends what kind of mass it is.if this is lymphoma
than growing dynamic is unpredictable and it can take from few months to few years. More important thing is to evaluate is there any danger for his heart function. It may also be mixoma which is rare heart tumor
but should always be considered. In every case,the right diagnosis should be established before treatment can begin.
Wish you good health. Regards