The main story line assuming that you complete the main story quests and enough side quests to be sufficiently levelled to complete the main story quests will take about 40 hours of continual play approximately. Every quest in the main story line can be completed solo, although like all MMORPGs there is a benefit to playing in a party.
PvP is very much influenced by character progression, as having greater progression in the game will result in having more abilities available, thus doing more damage or healing. PvP is possible at low levels but having more ability points and better gear will yield better results in the battlegrounds.
There are "dungeons"/instances in game throughout the story which require group play and there is a random dungeon finder available in game to find groups:
Again, these instances are not required for the main story, although they are all tied into the main story.
In contrast, there are also instances in the game throughout the story which are enforced single player as well, in which you enter alone to complete a task. Again these instances are not required for the main story.
There is not much of a requirement to ever join a guild, with the content currently available in game. Obviously being an MMORPG the game is improved by playing with friends but even for the solo play the quest progression and story line are definitely a lot of fun.