That is true.
It depends on these choices:
1. If you play
Vanilla or one of the
big mods (e.g. TAC, RaR, WTP)
2. If you have selected
"Custom Game" or
"Play Now".
3. Which
"Victory Contidion" you have selected.
So yes,
normal games in
Vanilla have a
time limit of
300 turns.
But if you
chose this path you
can deactivate it and play and
endless game.
(That will just be ended by winning WOI.)
1. Do not play Vanilla but
one of the big mods named above.
TAC is a good choice for
beginners.
- Carefully selected additions (not too many new features)
- Lots of attention to detail (great quality of new graphics, sounds, texts, ...)
- Very atmospheric and still "authentic" to the original spirit of Colonization.
After you feel comfortable with
TAC,
maybe check
WTP.

(The forum should write enough about those mods so you can inform yourself.)
2. Definitely
start by "Custom Game", not "Play Now".
3. In the
Game Options there is a
section "Victory Conditions".
Take care you
deactivate all other Victory Options except "Independence".
Voila, you got your endless game 
(That will just be ended by winning WOI and otherwise continue forever.)
If you do not
declare and win War of Independence before that counter ends
you lose.