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I'm confused on what the actual length in the Star Wars lore of how long the Jedi were the guardians of peace and Justice of the old republic.

In the original film Obi Wan to Luke states for over a thousand generations. This would be starting about the time of Knights of the Old Republic which makes sense.

However in Episode II, Palpatine states it to be a 1000 years which I've noticed some people here use.

Which is it?

Legends:

Palpatine was referring to the Ruusan Reformation, an event where the Republic was reformed and restructured. It occurred about 1,000 years before Palpatine made the statement. Essentially, a new Republic was created.

The original Republic began in about 25,000 BBY. The modern Jedi Order began in about 26,000 BBY. So the Jedi served the Republic for all of it's existance.

This was specifically created in order to prevent a continuity error.

From the linked article:

The Ruusan Reformation was introduced in the timeline in order to explain several differences between the Old Republic as presented in the comics, and the one which appeared in the prequel trilogy.

In the Expanded Universe, the Republic is well established to be extant for 25,000 years, based on Obi-Wan Kenobi's line in A New Hope that the Jedi had been "the guardians of peace and justice in the Old Republic" "for over a thousand generations." For a long time, EU sources were the only works set during the Republic era—most notably the popular Tales of the Jedi sagas. However, in Attack of the Clones, Palpatine mentioned the Republic had "stood for a thousand years." This at first seemed to conflict with both the C-canon EU history and the G-canon internal continuity in the films. However, the Ruusan Reformation—in which the Republic is reorganized following the already-established defeat of the Sith a thousand years prior to the films at the Seventh Battle of Ruusan—was invented by subsequent writers to give an alternate explanation on Palpatine's line, thus bringing all these sources into relative harmony through a retcon. It also explains how Sio Bibble could claim, in Attack of the Clones, that there had not been a full-scale war since the formation of the Republic.

Origin of the Jedi order is known to be 36,453 BBY. So, it should be 1000 generations (don't take this number literally), which was stated by Obi-Wan.

For over a thousand generations the Jedi Knights were the guardians of peace and justice in the Old Republic. Before the dark times. Before the Empire.

- Obi-Wan Kenobi

About 1000 years before Episode II, the Sith became extinct at the Seventh Battle of Ruusan (1000 BBY), where all of the Sith except Darth Bane and few more were killed. Palpatine might have referred this and you misheard.

For those who think Obi-Wan may have misspoke, there is pretty clear evidence that in the current canon the Jedi have been around for a thousand generations, not just a thousand years. In The Clone Wars episode "A Test of Strength" a group of Jedi younglings are introduced to a droid named Huyang, who has been instructing younglings in lightsaber design for a very long time. Then two episodes later in an episode titled "A Necessary Bond", at 11 minutes 7 seconds in, Huyang says "If I may, sir, I have instructed younglings for over a thousand generations, and these are among the best I have ever seen."

Likewise, the canon novel Tarkin says on p. 115 that "Tarkin suspected that even if the Jedi had restrained themselves from rising against Palpatine in his moment of glory, the esoteric Order had doomed itself to extinction. Where their flame had burned bright for a thousand generations, technological might was the new standard."

The Clone Wars episode "The Mandalore Plot" also had some dialogue indicating there had been a previous "Old Republic" which had fallen, with Pre Vizsla brandishing an unusual black lightsaber design, referred to in Ultimate Star Wars as a darksaber, and saying to Obi-Wan "This lightsaber was stolen from your Jedi Temple by my ancestors during the fall of the Old Republic. Since then many Jedi have died upon its blade. Prepare yourself to join them!" So, Palpatine's comment in Attack of the Clones that "I will not let this Republic that has stood for a thousand years be split in two!" may simply have referred to how long the Republic had stood since being re-formed after a previous "fall".

There IS a contradiction. There's no getting around it. It's simply a movie continuity error. Obi-Wan infers that the Old Republic stood for over 1000 generations & the Jedi were the guardians for all of this time. Palpatine infers that the former Republic had only been around for 1000 years. Out of universe it's a continuity error. In universe the only explanation is that either Palpatine or Obi-Wan made a mistake. No one's perfect.