Only Greenland (on achieving self-rule) and Algeria (on gaining independence from France) have left the European bloc, and the only legal pathway out of the current EU is through article 50, inserted in the 2009 Lisbon treaty. This allows a country two years to negotiate the terms of its exit from the moment it notifies the EU of its intention to leave. A Brexit vote does not represent that formal notification. Those playing for time will argue that six weeks must be allowed for legal challenges to the referendum result, so at the very least it might be best to wait for the result to be immune from legal interference.

But once the notification is triggered, negotiations cannot extend beyond a two-year notice period, unless all remaining EU member states agree to such an extension. It would therefore take just one member state to insist the divorce must happen at the end of the two years.

http://www.theguardian.com/politics/2016/may/31/what-happens-next-if-britain-votes-to-leave-the-eu

The form of any withdrawal agreement would depend on the negotiations and there is therefore no guarantee the UK would find the terms acceptable. The EU Treaties would cease to apply to the UK on the entry into force of a withdrawal agreement or, if no new agreement is concluded, after two years, unless there is unanimous agreement to extend the negotiating period.

During the two-year negotiation period, EU laws would still apply to the UK. The UK would continue to participate in other EU business as normal, but it would not participate in internal EU discussions or decisions on its own withdrawal. On the EU side, the agreement would be negotiated by the European Commission following a mandate from EU ministers and concluded by EU governments “acting by a qualified majority, after obtaining the consent of the European Parliament.” This means that the European Parliament would be an additional unpredictable factor in striking a deal.

http://openeurope.org.uk/today/blog/the-mechanics-of-leaving-the-eu-explaining-article-50/