I usually like to watch it weekly but there are times when i like to wait also. thats usually due to the fact of cliff hangers that i wait otherwise i go berserk and throw my laptop out my window.
For new series I watch it all at once because I enjoy having details and thoughts from the previous episodes fresh in my mind while watching the next ep. I don't do that for second seasons though because I'm familiar with the setting and I get too anxious over watching more of a series I already enjoyed.
Many of my friends hate waiting 1 week to see the next episode, and I somewhat agree with them if I'm not a great fan of the anime I'm going to watch, but there are some series that I just love from the first episode and I usally end up waiting for the weekly episode
I've seen so many animes by now and now that I've seen all that I wanted and I'm now waiting for the new ones, I need to wait and that sometimes is soooo frustating but oh well, I love watching them :P
I'm definitely more of an "all at once" person. Most of the anime I watch isn't ongoing, so I don't have to wait. So, on days when I have nothing to do, I'll just have an anime marathon in my room. My record is watching 18 episodes in one sitting.
After i discovered the wonderful world of anime streaming online, I stopped watching animes like Fairy Tail, One Piece, etc, and got used to animes that i can watch it all in one day (12-50 epis).
Mostly weekly. Because sometimes when I don't watch it weekly I get lazy watching it all together and end up not watching it at all. But sometimes I watch all together so that the frustration of waiting to what happens next is calmed :D
Most of the time it is all at once, I don't usually keep up with new anime until it is complete. There are a few, like Attack on Titan I am watching weekly.
I hate waiting weeks D: but sometimes I mean you really have to.
Although sometimes if it's a weekly thing, I'll wait a couple weeks, watch another anime, and then go back to watch a couple episodes at once :3.