Very low frequency instrumentation amplifier

Instrumentation amplifiers are designed to work down to DC, which is effectively what your application is. You'll basically need to look at all the sources of error (offset voltage drift will be key) and assume they can drift over their full range over one period of your measurement.

Typically, your ability to control the operating temperature of your system will be key to obtaining good accuracy.

Luckily, this is exactly the sort of thing in-amps are designed for, and they will often be spec'ed for things like temperature coefficient of the voltage offset, etc.

Edit

This is just to point out one more thing you want to look out for.

Here's the typical spectral input noise for the AD8235 StevenVH recommended:

You notice that the noise is increasing at low frequencies below about 10 Hz. This increasing noise at low frequencies is a very common (universal?) feature of op-amps and in-amps, called 1/f noise. The name comes because this noise source has an rms amplitude roughly proportional to 1/f. So you can expect this curve to continue increasing in a well-defined way as the frequency drops to 1 Hz, 0.1 Hz, 0.01 Hz, etc where you say your application is operating.

However, depending on the manufacturing process, etc, different amplifier types will have different levels of 1/f noise. You will want to look at this behavior as a key parameter for selecting an amplifier for your design. If some data sheet doesn't report the noise at all at low enough frequencies to show 1/f noise, that is not a part you want to use here. That said, the AD8325 does appear to be very good in this regard, with its "knee" at only 10 Hz.

Like The Photon says, they work down to DC, it's just that most customers don't use them for those very low frequencies, so they don't publish data on them.

I checked InAmps at Digikey with low slew rates, they will be better suited for your application. The AD8235, for instance, has a slew rate of 0.011 V\$\mu\$s, and the datasheet show graphs for noise, CMRR, and such down to 0.1 Hz.

InAmps work just fine at 0.00Hz, i.e. at DC. Like The Photon said above.

It's higher frequencies (above 3kHz-10kHz depending on the chip) that an InAmp may have problems with. CMRR falls with frequency. Sometimes, the signal is at sufficiently low frequency, but the interference is at high frequency (100kHz LORAN signal, for example). That still leads to reduced CMRR. So, low-pass filter the the InAmp inputs to remove the high frequency noise. Common mode choke or an RC is usually adequate.