Other than for any light which escapes 'as light' through windows etc., I can but presume that it must all eventually get turned into heat, since it cannot continue 'building up' within the building. Even if it is largely reflected by each of many (maybe white and shiny) surfaces it bounces off, I presume that at least some gets absorbed by each reflection, so that it will eventually all be absorbed, and turned into heat.We have debated what happens to light? I would assume if light hits a dark surface it is absorbed and turned into heat, so colour of curtains and drawing the curtains before switching on lights must also effect the room heat, even ignoring the heat lost by conduction and convection.
Kind Regards, John