Here’s the thing, which might be surprising… I never let myself “commit” a ton of time to an artwork unless I can already see the end result in my head. When I commit a long time to a painting, it means that there’s already a certain way I imagine the piece to look and only x amount of hours put into it will achieve what I want. If you have really good, well-thought-out vision and plenty of references and/or research to back it up, the hours of rendering work should come naturally.
Remember - time spent doesn’t automatically equate to great art. The danger of spending too much time on something is getting too attached to an idea, making you blind to any flaws; a poor initial composition is still just going to end up a well-rendered but poorly composed piece, and you won’t be able to start clean because you’ll think “I’ve spent too long on this to let it go.” Further, it’s much, much harder to take criticism on something you pour too many hours into.
I am in no way saying that being dedicated can’t lead to great results. Plenty of artists create only very involved and time-consuming pieces, and it’s amazingly admirable of them. But if that’s just not your thing right now, you shouldn’t feel bad about it! If you’re like me (short attention span!), mentally telling yourself “I have to spend a million hours on this piece! It’s gotta be BIG!” is a sure way to get discouraged and impatient unless you are incredibly enthusiastic about the idea. Don’t feel like you have to spend a ton of time in order to post it on your artblog. Listen to people’s feedback, look at inspirational work, and collect reference photos. Sketches, studies, and idle doodles are just as important, if not more important, than an artist’s “big pieces.” Each one will nurture your skills and make you more confident for the next piece, until one day you’ll have spent double, triple the time you normally spend and you won’t have even noticed!