I'm gonna push in here, cite my position as a member of a Norman reenactment group, give you a spiel and then go away again
As a prelude, all my knowledge is based on Anglo-Norman England from 1066 to 1215. Plate was not a thing then, and the best armour a knight would have would be a maille hauberk which consisted of an integrated coif and sleeves that ended in mufflers and continued down to the top of the knee, normally including a riding split. They'd then wear either maille hose (think tights made of maille) or chausses, which were effectively maille shinpads. Underneath this they wore a gambeson which was two tunics stitched together and stuffed with any sort of padding, but ideally horse hair. They used horse hair because they discovered it had a similar strength to iron thread in the same way spiders silk and steel thread does. They'd also wear a helmet, be it a nasal with a ventail (a strip of maille backed with leather to cover the nose, mouth and throat) or a helm with a faceplate, sometimes with a ventail as well. All in all this could weigh up to about 5 stone, but as it's evenly distributed across the body it's not too bad.
As to how effective this was, here goes. Maille was excellent at stopping slashing attacks, as the force of the blow is spread out amongst a number of links which themsleves spread the force to their neighbours, causing the hit to do nothing more than perhaps bruise. It was not however that good against piercing attacks, as the full force was put through a few links, causing them to burst or split, compromising the maille and in the worst case, actually leaving bits of broken link inside the wound. This is what the gambeson is for, it acts as a kind of spall liner, catching any shards, and also reducing the force of the blow some more. There are stories of crusader knights who advanced through arrow fire, getting peppered with arrows, but the combined thickness of their maille and gambeson actually prevented the arrows from reaching the man underneath. Finally, maille is almost useless against impact. It reduces the effect slightly, but this is wear the gambeson shines. Due to it's padding it spreads the force and acts as a shock absorber.
With regards to all the kit a knight would bring with him on campaign, he would have had a couple of spare shields and a fairly elaborate tent with bed and other comforts. However at the same time, knights were perfectly willing to go out on scouting missions or skirmishes and sleep rough with the common soldiers. Knights were also trained from the age of ten to be knights, and thus were perfectly capable of spending a whole day in armour and then fighting in it.