That's a good question. I'm guessing the longest time between official tours would be from:
2004-12-17, - STUDIO 287 - PARIS, FRANCE (end of UHH tour)
and
2007-07-07, - THE BRIDGEWATER HALL (MANCHESTER INTERNATIONAL FESTIVAL) - (start of White Chalk tour)
She said to the audience during the 2004-12-17 gig that that was the last time she would perform (or something similar) leading some to infer that she was done with performing. She probably did burn out during the UHH tour since she constructed the WC tour as a solo tour so that she wouldn't have to keep to the typically demanding schedule that's required when you have a band and full crew under contract.
You can view pretty much her entire performing history here:
http://www.baymoon.com/~thegarden/As you can see there, she did perform quite a few one-off gigs in between 2004-12-17 and 2007-07-07. I'll bet the longest gap between any performances was between:
2005-04-23, - CAMBER SANDS HOLIDAY CENTRE (ALL TOMORROW’S PARTIES)
and
2006-01-12, - THE ECHO (MORIS TEPPER W-PJ HARVEY) - LOS ANGELES, CA, U.S.A.
but there was this gap also:
1997-07-13, - SALZBURG FESTIVAL (JOHN PARISH, POLLY JEAN HARVEY & THE MARK BRUCE DANCE COMPANY) - SALZBURG, AUSTRIA
and
1998-05-12, - “LATER… WITH JOOLS HOLLAND” - LONDON, U.K.
So it looks like 9-10 months is about the max time she can go before she has to perform again.
That being the case, since she last performed on 2009-07-25, I predict she'll resurface sometime around May of this year (assuming things go well in the studio in April with the new album).