How to analyze a circuit's time constant with capacitors and resistors in series

[Kituro]

As the image shows, The capacitance of the circuit will be the two capacitors in series, which is 135nF.
So the time constant will be t = R * C = 520ohm * 135nF = 0.7us, but in the oscilloscope the charging time is obviously more than 3.5us(5t)

I've build a current source using an op-amp.This current source circuit is always used in DC.
But i tried to make it work in AC by simply adding two capacitors(to isolate DC offset).From the simulation, it seemd working properly.
Then I want to calculate the time constant of this circuit, but the result contradicts the oscilloscope's graph.
The op-amp is ideal, whose output resistance is 10ohm. And it has connected to +12V and -12V DC supply.
Fuction generator is set to output a 1kHz sqare wave at 5V(peak-to-peak).

Did I calculated correctly?How to analyze this circuit?
I'm a beginner in electronics, I'd be appreciate it if you can help me.

[Terry Bites]

The DC operating point for the opamp is not defined. The distribution of charge between the two caps cannot be defined. On the bench this circuit will act as an integrator and drift off to rail.

[Kituro]

Thank you for your reply.
I looked up some information about how to bias the op-amp, there are many solutions.
Could you please give more details?
Can I have a simple voltage-divider connected to the terminal or place a zener diode between the terminal and ground?