You notice when Its maxxed when the bonus ends with "000" or "500".
The Maximum depends on what buildings you have.
GuildLv3 216k GuildLv4 432k
Tier 3 325k Tier 2 144k
And what you need to max it depends on the strength of the opponent and your buildings. (The plunder bonus to allies on one's profile is taking the strongest possible opponent into the equation. But since you will most likely hit weaker opponent, the plunder and plunder bonus will be lower. No one knows the forumula, so noone can really calculate it completely. Once you hit around 200b in allies, this is negligible as you max out on any opponent.)
Hope that helps.