It would heavily depend on how fast you are expected to post, I figure - I think the GM mentioned something about roughly a post a week, but being fairly lenient with it? Just posting also requires a notably less effort than actually running a RP. 

I personally would naturally consider Angoriel a welcome addition (as you might or might not have noticed, I actually did get to posting a character of my own this time around, even if the CS is still a bit unfinished)... Whether it would be reasonable is another matter. The GM permitting joining in later (and looking at how we joined in later on the last time, I can't see why she wouldn't), I would perhaps suggest waiting till the end of September or so before you either join or do not... see how the RP takes off and what your workload and life in general would be like in practice. 

@SweetCinderella: Does the kind of the being she targets matter at all... Good question, actually. Mental strength has been the only factor which has been stressed repeatedly and pretty much everywhere as being something that matters with the beings she's based off on, so I'd say this is predominant indeed. 
But if I am trying to guess, then the more "mundane" a being is, the harder they would be to drain, with the exception of beings that are actually ethereal? The few mentions there are of it in the stories, they [beings she is based on] have been able to defeat magical beings quicker than humans typically succumbed (while they naturally only targeted humans and in rare cases other animals, the released ones sometimes went and defended people from supernatural beings). Might also have been for mostly plot-reasons, but who knows... I also sort of imagine beings who are very close to her own kind - nature spirits, specters, other (semi-)ethereal apparitions - might be marginally more resilient, as they generally were towards the powers of their likes, and often were chased away rather than eliminated when it came to conflicts?