Hello,
This is impossible to say as there are many things. You have to measure it.
Some things that are of importance:
How much is "some"? loss depends strongly on the fill factor and whether woven or non-woven reinforcement is used.
How much distance from antenna to PCB and Q factor of antenna? When the antenna is close to the casing, the casing will experience (strong) reactive fields. The stronger the E-field at the casing, the higher the loss. Field orientation (perpendicular or parallel to casing) affects loss.
What is the frequency? high frequency will mostly result in more loss, but high frequency also results in lower antenna Q-factor (given same volume available).
What is theaccaptable loss due to the presence of the material?
If you want to measure it, you need to do impedance measurement (as the plastic will detune the antenna) and radiation measurement to assess the losses.