How often is he bluffing? Often enough to call? I am stressball...

Full Tilt Poker Game #5564163119: Table Bolt (6 max) - $0.10/$0.25 - No Limit Hold'em - 10:45:31 ET - 2008/03/09
Seat 1: stressball10 ($30.05)
Seat 2: boagmeister ($15.50)
Seat 3: Soultwister ($28.85)
Seat 4: grtdadda ($26.20)
Seat 5: IHateToBeOnTilt ($41.30)
Seat 6: horstharry ($8.40)
grtdadda posts the small blind of $0.10
IHateToBeOnTilt posts the big blind of $0.25
The button is in seat #3
*** HOLE CARDS ***
Dealt to stressball10 [9s 9h]
horstharry calls $0.25
stressball10 raises to $1
boagmeister folds
Soultwister folds
grtdadda calls $0.90
IHateToBeOnTilt folds
horstharry calls $0.75
*** FLOP *** [7d 3d 4h]
grtdadda checks
horstharry checks
stressball10 bets $2.50
grtdadda calls $2.50
horstharry folds
*** TURN *** [7d 3d 4h] [8s]
grtdadda checks
stressball10 bets $6
grtdadda calls $6
*** RIVER *** [7d 3d 4h 8s] [7s]
grtdadda has 15 seconds left to act
grtdadda bets $4.50
stressball10 has 15 seconds left to act
stressball10: i'm paying you off, but you suck
stressball10 calls $4.50