I think what Kell wrote above is key: is this agent's style/strategy working for YOU?
I parted ways with my first agent for the very reason you wrote: I felt she was quitting on my manuscripts after only six to ten subs. She was always pushing toward the next manuscript, and it most definitely rubbed me the wrong way, considering the six to eight months of work I put into each, working days/nights/weekends/holidays.
I wasn't willing to throw my manuscripts around that easily.
When I would ask to sub another round, she would dance around the subject. When I finally asked for a phone conference to discuss subbing issues, she kept putting me off, until I finally had enough, and I terminated our agreement.
I wanted an agent who was willing to sub more rounds, and who was as hard a worker as I am.
I hear you loud and clear: all those hours of work, all those sacrifices you make from life, friends, family time. You want an agent who is willing to give your work a serious effort -- as much effort as you put in. Those agents are out there.
As for how long it should take an agent to read a new manuscript, one to two weeks is wonderful, and achieved by some agents. Two to four weeks may be the average for others.
However, an agent should be reading clients' manuscripts before referrals, contest, or slush pile manuscripts, also. While there may be an exception at times, say, if a slush pile ms has other offers, your ms should come first and be read in a timely -- two to three weeks -- manner.
But again, the above is in a perfect world. And we need to balance out the pros and cons and remember agents are only human, and usually pretty busy. But it doesn't hurt to know what does or doesn't work for you, and to look for an agent that can offer you what you need most. I know, for me, waiting to hear back on a new ms is excruciating. So, an agent that took six weeks or more would only serve to make my life (Satan's home) -- aka, wouldn't work/be a good fit, for me.
Good luck to you, and most of all, do what you need to do for your own sake -- your own mental health. A bad agent fit is a miserable thing. This is your work, your career, your paycheck. The agent should be willing to work with you in a way that honors both sides of the business partnership.