I would guess it depends on what I want to do with the game.
Say I play on a level I find that I handle well, and want more of a challenge. A thing to do then is perhaps to start with bad terrain, so I have to work more to get ahead in the game.
If I try for the One City Challenge - a variant of gameplay where you are only allowed to own 1 city for the duration of the game, and can try to achieve a 1 city cultural victory, a spaceship victory, or conquer the whole world whilst razing all cities. Then I would be more picky about the start: a good idea is to have a source of fresh water - a lake or a river. You get extra commerce on river squares, so the more river squares that actually is in the start position, the stronger the start, I reckon.
Then there is the issue of grassland, bonus grassland, cows, wheat, bonus resources
In conquests, forests can be useful early - you can chop them to give good help in terms of shields. So a forested start isnt necessarily a bad start.
If I am lucky enough to get more than 1 cow (I got 3 once), I ll look into developing what they call a settler factory- where I produce settlers every 4th turn at least. To do that, you could use a combination of different types of terrain as well, but I tend to have more faith in the game with cow a start for some reason.
Jungle or desert starts is most likely to be a "start new game" attempt. Unless I have a civ with the agricultural trait, I wont bother (with the exception of the above mentioned extra challenge)
Also, it is important to look beyond the initial 9 squares. If I feel that the initial 9 is worthy of settling on the spot, I do so, and check the total 21 squares my city would be using in the future. Even then I may reload, if I feel the start is too weak.
Hope this was helpful for you

Good luck!
I am mostly a regent player myself - a level I feel I handle well now.
Monarch is my new arch-enemy, and I tend to chose stronger starts when playing that level, than when playing regent.
Edit:
Woops -forgot to give an average
Probably 10-20 seconds, more if I cant remember the correct civ traits, less if I am looking for something specific.