I almost never do a two hour date the first time. It depends on the price to a degree.  If the two hour price was reasonable and the lady was well reviewed and my type I might give it a try.

I found for quite a while that I desired more than an hour...  1.5 was about right if available.  As I got more experienced, an hour was enough.  
Personally, I book an hour...  Faves that I know I click with, maybe more.  
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I very much prefer 2 hours or longer.  Feels to me that one hour appointments it's time for her to leave just as it's getting interesting.

I know that wasn't your question, but the sentence "I am using agency, since I am at hotel and no car" didn't make sense to me. Why not see an indie outcall (if you want to, not knocking your agency ladies)?

Anyway, to answer the question you actually asked. Two hours is nice in that it gives you a lot of time to relax around each other -- which will be important your first time. However, it is more of an investment both financially and time-wise. Perhaps you could see if a happy compromise, a 90 minute option, is available from the 1 hour lady.

Start with 1 hr. If the lady is not booked you can call the agency if vibes are going good and extend.

I prefer the 90 minutes to two hour time frame for a first visit. Admittedly it is a bit more of an investment in terms of time and money. and it can be a drag if you do not hit it off with the lady. But I find a two hour session gives me time to chat with the provider, drink a litte wine, and generally get comfortable before we "get comfortabe." Also, if you have to drive a distance, a I often do, to see the ladies it gives you time to shower without feeling as if you are missing out. Finally, I find I can usually "drink three cups of coffee" before time expires, which gives me a sense that it was worth the expenditures. But as others have said, its personal preference.

My first session ever was a 90 minute affair I knew I'd need a little extra time to relax.  I prefer longer sessions to set the GFE mood.  I generally don't do anything less than 90 minutes now, but if time permits I prefer real "dates" of 3-4 hours with dinner or lunch before hand.