Well, Norway could pull a Lithuania, but I don't see it being nowhere more successful, at the contrary.
Besides, Christianisation offered a lot of interesting features : it basically allowed a leader to get his domination acknowledged by Christian powers, whatever for ruling a part of their lands (Rollo in Neustria, Godfrid in Frisia, etc.) , or not to be threatened on the land he already ruled over (as Danes).
Such "diplomatic reckognition" was also accompanied by "peer pressure", critically when Christianisation promoted a political model that was based on the importance of the king over assemblies (less by religious drive itself, than association of Christianity and imperial/royal models).
But yeah, Norway, critically if it remains divided as Sweden was, could have a delayed Christianisation or know some form of expedition as it happened against Lithuania. In this case, I would see Danemark being the main benefitter.
Presuming everything goes as historicaly as possible, what is the realistic time estimate before preasure is too much and they have to do it?
Well, without a catastrophic collapse of Europe, I would say somewhere between the XIIth and the XIIIth centuries before formal Christianisation, maybe up to XIVth for an actual complete Christianisation.