PTC thermistor simulation in Proteus
The time constant is variable.

1s if  It/Int= 5,
6s if It/Int=2.5 and
35s if It/Int=1.3 which is the 

From external self heating hot air it appears to be >>35s, which is the lowest trip ratio given for 5mm and all sizes.

It is a self heating PTC not a temperature measuring sensor.
The time constant is a function of the Trip current It over the non-trip rated current Int. or It/Int

The Dissip. Factor (DF) for the 5mmD part is 7.3 (mW/K) therefore the Rja= 1/DF = 137 ('C/W) 

The voltage derating is 100% from  75'C to 175'C therefore the trip temp is 175'C with a 100'C swing .  But if the Pd were known to be 0.75W at trip temp, the Rca would be 100'C/W/0.75W = 133 ('C/W) but we do not know this Pd. as it is not exactly constant


  I want to ask 2 more questions. 1)If R=11 ohm in 25 °C and Iolmax =1500 (max overload current),then How can i calculate lolmax in 50 °C ?What is the formula? 2)If R=11 ohm in 25 °C,then how can i calculate R in 50 °C and in other temperatures? What is the formula? 


You have to know how to create a formula from the curves given in your datasheet on pages 2 and 5. Now since these are not specified as a formula, you need a really good reason to do this and a better understanding of uncertainty and errors in this calculation.

If you see the curves on these pages, you may be surprised to learn that the 

PTC actually has an NTC curve below 85'C

and then it flattens out and turns upwards to PTC.


The slope from 25'C to 75'C = k (Log R)/T   where k is the negative NTC coefficient which is never given.  So why do you need it when you have the curves on p 2,5?