I'm not an accomplished indoor grower. But, I've studied everything I can and have some experiencial learning (you know, from making mistakes ). Since no one else has chimed in, I'll offer my two cents.

Assuming your medium is nute free...

If they're auto's they are past ready for some light nutes (1/4 strength). (This I know from personal experience)

If they're photo's, they should be ready for a similar dose as long as they have two or three full sets of fan leaves.

The consensus seems to be to add light nutes every third watering. Some folks increase the nutes in small increments but, you have to be very vigilant for signs of nute toxicity.

Most of the people I've seen here that seem to be esteemed as experts, have a tendency toward "less is more"... better to suffer a slight loss in yield than to have a crop destroyed or severely compromised due to an over aggressive nute schedule. :thumbsup:

Anyway, I hope this helps. All the best with your grow.

Are you growing auto's or photo's?