Most people find that light therapy in the mornings works best so, if you are just starting out, try this first. After a week or so, if you're not feeling much different, try introducing another session in the afternoon.
If you don't have time in the mornings, then using a light in the afternoon will still be better than not using one at all. Try not to use your light late in the evening though (i.e. within 3 hours of going to bed) as you may find it difficult to get to sleep.
If you have trouble waking up in the mornings then you should definitely use a dawn simulator. If this isn't enough, then use a light as well even if you only have time for a 10-minute rather than 30-minute session, and again top up later.