how long before I transplant

Ive had a plant in veg in a solo cup for a little bit over a month now, it stands about 4 inches tall and has seems to be very strong. I just want to know when I should transplant it into a bigger pot.

Thanks,

Spinny

now, i would move to i one gallon a week or two later in a 5gal. then switch to 12/12. i try too keep an eye on the bottom for roots when i see them i transplant.