Not common at all as far as we know today. Women taking up arms was probably a desperate last effort rather than a common thing. It likely happened among the Varangians during the Siege of Dorostolon but that was again a desperate situation. Freydís Eiríksdóttir supposedly took up arms during the Vinland expedition but that is not exactly the most reliable of sources and she is only said to have scared the enemy off, never actually fought anyone.

Vikingaliv by Harrison, D. & Svensson, K mentions quite a bit about the phenomenon, I do not believe it has been translated however.