All TFT monitors fade over time which means they won't be as bright as they were as time goes by.
Typically they fade around 40% each year at the same brightness settings (as do car headlight bulbs). This is due wear on the fluorescent back light (CFL) and yellowing caused to the internal plastic screen. This natural fading can make your images harder to perceive by participants. Even if you leave you settings the same in 12 months time the images you present may be 40% harder to see and react to.
Results of Normal Temperature Backlight Test Data from Sanken Electric Co Ltd, Japan (www.sanken-ele.co.jp)
CCFL backlight manufacturers for major laptop brands.
Again this fading will vary between make and model and between two apparently identical TFT monitors at the same brightness settings. This is because it is dependent on usage as well as age. Replacing a single monitor for a multi-display array can mean that all the monitors will need to be exchanged.
This will be an issue in experiments where more than one TFT monitor is used or the monitor is used over a prolonged period to rerun the same or modified experiment.
Ideally all display devices should be calibrated about once per month using a professional calibration device such as the one shown in the video. This applies not just to brightness but also to colour hue and intensity.