Need advice ono a Tube preamp

If I buy a tube preamp new then:
1) how long does it take to burn in
2) how often do you need to change tubes
3)will changing the type of tubes affect the sound much/little and if so are there certain tubes that are considered to be better than others?
Sorry for all the questions but I have really no idea about this stuff and am trying to lern before I buy.
Thanks for all the input.

TAD-150. Read the reviews...

I see some tube preamps have one tube,some 2 and some 4.I only need a line stage preamp.I would also prefer balanced outputs.As I am trying to warm up the mids and highs a bit is it better to have more tubes? Or does the number of tubes even matter?

You can accomplish what you want with a tube preamp. However, rather than focusing on the number of tubes a preamp has I would rate your priorities as follows:

1. Design, including how the tubes are used in the signal path and/or power supply.
2. The type of tubes being used. Each tube has certain characteristics that "color" the sound. Look for one that has the characteristics that match your sound preference.
3. Abililty to tube roll. This is not so important to some, but to me having this flexibility is a nice-to-have feature. You'll want to know what NOS options or alternative tube types will work with the preamp.

A lot of people are hot on the Modwright preamp right now. It might be worth a loook. Me, I'm biased, but I'd recommend you look into a Joule Electra LA-100 MkIII. Here is one listed on Audiogon:

http://cls.audiogon.com/cgi-bin/cls.pl?preatube&1138632048