Last I read it (some months ago), the data usage was unlimited, but: say in the case of a "4GB" mention. The 4GB would be received at whatever max speed you could attain at given moment. After that, it went into a crippled mode (speed drastically slower until the refresh date rolls around.) All of that falling under 'fair usage.'
- - - - - - Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.