You need a board meeting whenever you need to take a decision which requires board approval. To some extent, this will depend on the company's articles of association and how much authority has been delegated. You must also hold a board meeting if any director asks for one.
In practice, you will need at least one board meeting annually to approve the annual accounts. As a director, you also need to ensure that the number of board meetings you have (and the matters on the agenda and the way the meeting is conducted), who you are complying with your duties as a director - for example, that each of you is promoting the long-term success of the company and exercising independent judgement. Most companies hold regular board meetings - quarterly or monthly - to review management accounts, discuss strategy and so on.
Of course, these meetings can be supplemented by informal meetings of some or all of the directors for discussions where formal board approval is not required.
If you cannot physically attend a board meeting, then some articles of association allow directors to take part by telephone or videoconference, or for decisions to be made by every director signing an identical 'written resolution in lieu of a meeting' rather than attending a meeting.
