I usually apply this product at night just because it gives my lip an extended period of time to soak in the product. This product can definitely be applied during the day over a lipstick and they even have products in different colors. But during the day activities such as eating and drinking will fade this product. If I do not wear makeup I'll apply this product throughout the day. The choice is really up to you.
If I'm in a situation where I can't reapply chapstick/ lipstick or gloss for an extended period of time my go to is this mask in smashed. It keeps my lips coated in moisture all day and adds a pretty hint of color and glossy finish.
I normally apply it only at night but I'll wear it during the day when I'm having lip dryness. I only do daytime application if I know I won't be eating anything for a few hours, because it would get wiped away with a napkin. It is SUCH a good product for severely dry lips!