How many signatures for referendum?

Occasionally critics of Direct Democracy remark that proposals to launch a binding referendum do not get the support of sufficiently large numbers of signatories. But it has to be considered that collecting them is an uphill task and the launch of such a Campaign often has to be at the expense (in money and time) of volunteers. The trigger point to make a referendum proposal binding must therefore set at a point that is not too low (in order to avoid nuisance proposals) and not too high (so as not to make most or all proposals ineffective).