How long for ultrasound referral to arrive?

Small lump in the parotid area appeared a few weeks back. Dr told me to go back after my holiday if it was still there. Went back today as it is indeed still there and has grown in size. 

He has referred me for an ultrasound, I'm just wondering if anyone knows how long it will take? I'm very keen to get it over and done with as I'm very anxious about it as I'm sure most people are. Would love to know a kind of timeframe though. Thank you.