Follow the shadows
Look at the proposed pot position at different times on a clear day. Nearby buildings, balcony roofs and trees can shorten direct sun even when the space looks bright. Repeat the observation as the season changes.
As a general starting point, many culinary herbs do well with around six hours or more of direct sun a day. This is a broad guideline, not a guarantee: local heat, reflected light, cultivar and water supply matter. Thyme prefers a sunny position; mint can tolerate partial shade.
Notice the exposure too
A sheltered corner and an exposed railing can offer different growing conditions on the same balcony. Watch for wind, hot paving and reflected heat from walls. Keep pots secure and within the balcony boundary.
Move newly purchased plants into stronger outdoor exposure gradually. If leaves wilt, check the potting mix before assuming the answer is more water; use the watering note to separate dry roots from other stress.
Reference
- University of Minnesota Extension: growing herbs (sunlight and general culture; adapt regional advice locally).