Low power LED for autonomous project
How long is a piece of string? I'm not being dismissive - I really mean that the answer is "how bright do you want it?". A couple of things that will help:


LED brightness is very roughly proportional to current, not voltage, so concentrate on that parameter when evaluating LEDs.
Lumens are less useful for comparison sake until you get into high-power LEDs - the measurement particulars and consistency of a lumen measurement in a low power LED (ie. one not used for indication, not illumination) are too complex to be useful.
There are lots of other factors that can have a bigger effect on visibility than brightness: distance from observer, contrast with surrounds, light pipe or diffusion layer, colour.
For low-power LEDs, red is generally brighter at the same current level.
You really need to suck it and see. Try a standard 3mm red LED and drive it at 5mA, then 10mA, then 15mA.
If you need to attract attention, then you'll struggle without a high power LED. If you only need to ensure that someone can tell whether the LED is on or off, then you can probably get away with a 3mm red LED at 15mA.
Flashing is much easier to spot than steady.