Material with fairly high resistivity, but allows flow of charge
To oversimplify a bit, the discharge will take place with a time constant (measured in seconds, hours, microseconds etc.) that depends on the product of the capacitance and the resistance. 

The capacitance of an conductive object in air is mostly dependent on its surface area. An isolated sphere of radius R has a capacitance of \$4\pi\epsilon_0R\$. 

The resistance is dependent on the surface area of contact and the resistivity \$\rho\$ of the material. 

So, though geometry certainly enters into it, one can say that for a given geometry, in air, the discharge time constant is inversely proportional to the resistivity of the material. Resistivity is a fundamental material property that you can look up in a chart or on the internet. 

Remember that discharge is an exponential decay. It does not make a lot of sense to say it is completely discharged, but after 10 time constants maybe 50 parts in 1,000,000 of the original charge remains (\$e^{-10}\$ if  you want to work it out).

Take care that materials which are decent insulators tend to have a wide range of possible resistivities. 

Eg. 


  Glass: \$10^{11} \text { to } 10^{15}\$ Ω·m
  
  Hard Rubber: ~\$10^{13} \$ Ω·m
  
  Acrylic: \$2\cdot 10^{15} \text {  to  } 1.4\cdot 10^{16} \$ Ω·m


So it's quite easy to get an order of magnitude or two difference in resistivity. Resistivity of insulators tends to be very dependent on temperature. Glass at 1500'C is about as conductive as damp wood, about 250 billion times more conductive than at room temperature. 



The above assumes that volume resistivity dominates. If the surface is coated with something that is relatively conductive then things change dramatically. There are  various types of anti-static spray that are designed to leave such a residue. Some of them are closer to metals, and others are so high in resistivity  that they cannot be measured with an ordinary multimeter.