Now? Even if you want a referendum "now", now would presumably mean in about six months or something. Now (or rather, then) is not the time to have a referendum. Firstly, because the future of the UK itself is in doubt, with the impending Scottish referendum on independence. Secondly, because that would place a referendum very close to the General Election next May. Thirdly, because there is still the potential that some real and meaningful negotiations will take place before the intended referendum date that change the picture.
Imagine the chaos if at around the same time the UK left the EU and at the same time broke up. It would surely be hideous for all parties concerned, as presumably Scotland's hopes for an Independent future within the EU would be destroyed, our own efforts to maintain two separate negotiations would be a major distraction from everything else the government has to do, etc. etc. I think most people should see that a snap referendum would have no winners in practice. Those in the "no to the EU" camp might lose doubly, not just the vote, but also the chance to have any say for a generation and the chance to force the government into (at least trying to) changing our relationship with the EU. Even if the "no" camp wins the vote it might be a Pyrrhic victory for the reasons described above and more. Since it also seems as if Scotland is generally more pro-EU, a referendum to leave it might just push the scales in favour of their own independence, so those in the "yes camp" might win only to see it being too late to save the UK. And for that matter imagine the horror if Scotland were still able to vote in a referendum to leave the EU shortly after they'd voted to leave the UK.
It's asking for political and economic chaos to rush into a referendum now. Instead we should just hope that the Conservatives finally honour their pledge to hold a referendum, and that Labour make a similar pledge and keep it, so that we can have a vote in 2017. Before then, there would be time to at least try to renegotiate our terms of membership. Successful or not, surely it's an attempt worth making before deciding to leave. Usually people at least try to save a broken relationship before suing for divorce.