What is the rule of thumb for land/taking-off on a sloped runway?
What does your POH/AFM says?

Ignoring winds and other factors, it depends on effective runway length, not just runway length. If you have a runway long (really long) enough, you can takeoff/land in either direction.

As mentioned here:


  ... advantage or disadvantage of a sloped runway is that a 1% runway
  gradient — an increase or decrease in altitude of 10’ for every 1000’
  of runway length — is equivalent to a 10% increase or decrease in
  effective runway length ...


Example:

Runway length is 1500' with 3% up-slope gradient


Landing uphill on it will give us an effective runway length of almost 2000’ (1500’ x 1.3 = 1950’).
Landing downhill on it will give us an effective runway length of just over 1000’ (1500’ x 0.7 = 1050’).




Just for reference, the runway (6: uphill; 24: downhill) at Lukla Airport has 12% gradient and is 1500' long.

Effective Runway Lengths (using the formula above)

 6: 1500' x |(1+1.2)| = 3300'
24: 1500' x |(1-1.2)| =  300'


Hence, hypothetically speaking, if runway 24 is 48,400' long with no wind, A380 can takeoff too.