Provided your target fails his saving throw, the description of Lesser Geas states that it lasts until the target completes its specified objective or, should the task not have a definite end to it, 1 day per caster level. It does not, however, say that the caster level limit takes priority over the "until goal is met" limit. Could it then be possible to force a target to perform a continuous task for longer than 1 day/level by specifying a duration to perform said task longer than that, i.e. "Stay here for 6 weeks" or "remove one brick from this wall every day for a year" or "go one consecutive year without thinking about harming me, my family, or my companions"?
I couldn't find an official errata, clarification, or FAQ for this spell, so I'm going to use the spell wording and my own explanations as best I can.
If you give a quest that has a definite end, the spell says explicitly:
The thing is that nothing says you can specify how or when the creature can do the quest. You're giving instructions to do something, but this is not total mind control where the target is mindless and just does exactly what you say. A creature under a Lesser Geas is still independent and free to decide how to carry out whatever task you've given.
As a GM I wouldn't allow most of your examples for the following reasons:
As per the spell:
Just standing there for six weeks would require someone else to bring food and drink to the character, or they die of starvation (assuming they can't create food, but most characters can't). It's also not really a service, although "guard this" might be. In the case of "guard this", I'd consider it an "open-ended task" per the spell description and use the duration limit.
This is specifying criteria that the spell doesn't allow. You could say "tear down this wall", and the geas would be in effect so long as it takes the character to accomplish that task (assuming it's a wall they're capable of tearing down, like a wooden fence). By saying to only remove one brick, you're also trying to word it such that the character can't accomplish the task you're giving them, and I'd again enforce the limit.
If it was "tear down this wall" and the character has the means to do that, I would consider that valid. If it's just someone being asked to tear down a castle, I'd consider that something they can't accomplish on their own (again per the spell description) and would enforce the duration limit again.
I'd argue that "not thinking about harming me" is not a service, course of action, or activity, and thus not a legitimate use of the spell. "Stay away from me" or "protect me" would be, but those are open-ended and I'd enforce the duration.