Most cacti and succulents like bright sunny locations and light is vital for flowering so generally speaking the brighter the better! If you choose a windowsill, choose one that faces south or west as this will give the plants the best light. Some species will thrive on a windowsill with occasional sun though they are unlikely to flower.
All species need light. Do not put plants in a room with no windows!
What happens if my plant doesn't have enough light?
Cacti and succulents behave strangely when they don’t get enough light. Often, you’ll see discoloration in in your plant if they need more light – deep green will fade to pale green, and bright pink, purple or yellow colours will often revert to just plain green.
Too little light also affects the growth habit of your plant. They will try to reach for light, often growing long and spindly. Cacti and succulents that normally grow in rosettes, like Sempervivum and Echeveria species, may suddenly start growing tall – literally reaching for more light:
What happens if my plant has too much direct sunlight?
Your plant will start to burn, generally appears as browned or calloused flesh on your cacti and succulents. Looking for discoloration, especially on the side of the plant facing the window, is your best bet in identifying burn. The burned leaves or flesh will also get a rougher texture than the rest of the plant.
There’s no way to repair burned leaves once it happens; you can prune them off or just simply adjust the environment so your plant receives more appropriate light.
This cacti is currently suffering from light growth, as you can see it's trying to reach for more light