duration geiger tube

Hi All,
i read something about the duration of the tube (SBM20), someone wrote that the reading is reliable till 1M ticks, others wrote that the duration is eternity.. do you have some deterministic value?
thank you for your support.
Adriano

SBM-20
Parameters and characteristics .
Life (pulses) at least 2*10^10

SBM-20, SBM-20U
HARD BETA and GAMMA GM DETECTORS

GENERAL SPECIFICATIONS
Gas Filling Ne + Br2 + Ar
Cathode Material Stainless Steel, 50 mkm
Maximum Length (mm) 108 / 101
Effective Length (mm) 91.0 / 83.5
Maximum Diameter (mm) 11
Effective Diameter (mm) 10
Connector Pin
Operating Temperature Range 0C -60 to +70

WALL SPECIFICATIONS

Areal Density (mg/cm2) 40
Thickness (mm) 0.05

: ELECTRICAL SPECIFICATIONS
Minimum Anode Resistor 1.0 megohm
Recommended Anode Resistor 5.1 megohm

Recommended Operating Voltage (volts) 400
Operating Voltage Range (volts) 350 - 475
Initial voltage (volts) 260 - 320
Plateau length (volts) at least 100
Maximum Plateau Slope (%/100 volts) 10
Minimum Dead Time (at U=400V, micro sec) 190
Working range (mkR/s) 0.004 - 40
Working range (mR/h) 0.014 - 144
Gamma Sensitivity Ra226 (cps/mR/hr) 29
Gamma Sensitivity Co60 (cps/mR/hr) 22
Inherent counter background (cps) 1
Tube Capacitance (pf) 4.2

Life (pulses) at least 2*10^10
Weight (grams) 10 / 9

Really Thanks for the useful information SonarFlash!
Regards,
Adriano