The amount of guild experience awarded varies. It is depended on specific factors... (Source)
Guild xp for boss kills is scaled on a few factors.
- Boss level
- Normal / Heroic
- Amount of guild members present (more xp for raids, since they are limited to once a week)
- 20/25 - 25 player raid
- 8/10 - 10 player raid,
- 3/5 - dungeons
- 8/25 - Old 25 and 40 man raids (Vanilla, TBC, and WOTLK)
Straight from Blizzard's Mouth (Source)
The amount of experience gained scales, with 3/5 guild members granting 50% normal guild xp, 4/5 giving 100%, and 5/5 giving 125% guild xp. The amount gained from each boss scales with your level relative to the boss's level, with Heroic bosses worth even more (1.5x). It pays off to have a full group of guild members!
Anytime you do something that awards Guild Experience (or would, were your guild not at the cap), you'll earn Guild Reputation, up to the cap. Guild Rep gained is equal to guild XP generated/365 (Guild XP is 1/4th of the XP your character gains, or would gain if at level cap).
Phrased another way:
Guild Rep = Experience/1460
If you want to gain guild rep fast, the best way to do it is far and away by running Heroic 5 mans. It's fairly easy to cap for the week after just a few runs.
Reputation from a 25 man Raid Boss on normal mode is somewhere between 56 and 58. The exact fractional value is, unfortunately impossible to determine due to the way WoW displays fractional rep gains. (that is, it doesn't, but it does track them.)
The weekly cap on reputation, as pointed out by Raven is 3500 reputation. This cap increases to account for any +rep gain modifiers such as the Human 'Diplomacy' racial trait, or the 'Mr. Popularity' guild perk (both ranks). (i.e. for a Human in a Level 12 guild, the weekly cap is 4200).
Best Answer
How guild experience works
There are many methods of gaining guild experience;
Experience in raid and dungeon groups is granted provided you meet the requirement for the number of people in the party. For example, with dungeons you will get get scaling experience based on the number of people from the guild that are in the party as follows;
For raids, the number of people required in the group from the guild is a set threshold, and simply exceeding this threshold will provide the bonus;
For arena, the whole arena group must consist of members of the guild to qualify for guild experience.
Completing quests will provide experience to your guild for each quest completed, the amount of guild experience gained is approximately 60k experience per quest handed in since the release of Mists of Pandaria. This means that you can get a guild to level 25 by completing around 14100 quests.
Guild experience required for each level
As there is no cap on experience gain anymore, there is no "minimum" amount of time that it takes to level a guild to level 25 as there was at the start of Cataclysm.
Source on WoWpedia