The 50 days, obviously during a long war they may be enlisted longer, would be as and when their lord called them up. And so a Knight would only receive pay for the days they were called up, the rest of the year they live on the taxes from their small fief and whatever other activities they took part in. Many tried to marry rich heiresses for the wealth. And with the Knighting I BELIEVE (Though I may, and am happy to, be wrong) was carried out by anyone who carried a title higher than a knight's, so someone who had land to give as a fief.