When we need to break circulation after mud in static condition, we need certain pressure to break mud gel strength. You may see that higher gel strength of mud, the higher pressure is required breaking circulation. So this post demonstrates how to determine pressure required breaking mud gel strength inside a drillstring.
Formula to calculate pressure required overcoming the mud’s gel strength inside the drill string as follow:
Pgs = (y ÷ 300 ÷ d) L
where Pgs = pressure required to break gel strength in psi
y = 10 mm gel strength of drilling fluid in lb/100 sq ft
d = inside diameter of drill pipe in inch
L = length of drill string in ft
Determine pressure required to break circulation inside the drill string by using following information
y = 12 lb/100 sq ft
d = 3.32 inch
L= 11,500 ft
Pgs = (12 ÷ 300 ÷ 3.32) x 11,500 ft
Pgs = 138.6 psi
Therefore, approximately 139 psi would be required to break circulation inside drill string.
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
When mud is kept static for a long time(during logging) i think geletation will increase with time(not sure) so what changes one should make in this equation to consider time factor