I went on a beta blocker for AVNRT about 3 weeks ago. My doctor said it would take about 2 weeks for me to really settle into it, and I think he was right. I had some small SVT attacks the first week and haven't had any since then. It took me about 2 weeks to go back to fully functioning at school and work though because it made me really tired and dizzy at first. I'm pretty much back to normal now though.
Maybe I'm wrong, but I think my doctor told me that it was best to take my beta blocker routinely. I'm on 25mg twice a day. He told me I could take an extra tablet if I had an attack, but I haven't really needed to. Perhaps you should continue to take the beta blockers regularly and it'll better control the attacks.
Good luck with it!