What rule should I use , if I have to set my intensity length. I have turbulence intensity of 1-5% , so my question is have to set the turbulence length scale, do I take as the inlet length for inlet and outlet length for outlet??

To set a length scale you need to know a characteristic dimension i.e. is there a grid across your inlet in real life?. If so then take 10% of the length of the grid spacing. generally "the rule" is 10% of a characteristic dimension.

As an alternative you can set the hydraulic diameter, but based on the grid dimensions.

Do this for the inlet. The outlet length scale is only relevant when you have reversal of flow, set it to the inlet value.

For internal flows, you can take 5 - 10 % of the hydraulic diameter as your turbulence length scale when you deal with a fully developed turbulent flow at inlets. Or you can directly specify the hydraulic diameter.

For external flows (e.g., far-field), I normally use the turbulent viscosity ratio (nu_t/nu)rather than the length scale. Because there's no convenient length scale to use in the case of far field. Typically, a small value order of 1.0 (1 - 10) is normally use for the viscosity ratio.

These are just general guidelines. Yet, our hope is that the results are not very sensitive to the inlet boundary condition for the length scale as long as it is way from a reasonable range.