Both my sync cards worked on the 91st day everytime until the last one, when I hit their internal limit for sp cli's on my Care Credit. Pretty sure I've still got some room on my Amazon store card, but we'll find out on March 13th!
So... when you're looking to build credit, reduce util, and you're looking for SP CLIs, How much do you generally find yourselves asking for with these guys? 10, 20% more at a time? I mean, they can't double it forever.
I wouldn't worry too much about how much to ask Sync for. I've never received the amount I asked for and only been denied once. Everytime I went low, they gave me more than I asked for (with the option to refuse) and every time I went high, they gave me less (again, with the option to refuse).