Note From the Manual:
Simple Attributes

Simple attributes are just regular old attributes that can be set to a single numeric value, or possibly a set of numeric values. For example, the timeout attribute consists of a single numeric value, for example /timeout = 1000. The size attribute consists of two numeric attributes, for example /size = (200, 300).  Inquisit allows you to set these attribute values to either a constant number such as "5" or a numeric expression such as "sqrt(5) + 22 - text.target.currentitemnumber".  

As an example, imagine you are running a task in which subjects try to answer as many questions as possible in 5 minutes. When their time is up, the current trial is interrupted and no more trials are run. One way to achieve this is to set the timeout of each trial as a function of the time elapsed since the task started. This might look like the following:

<trial timedtaska>
/ stimulustimes = [1=question]
/ validresponse = ("a", "b")
/ correctresponse = ("a")
/ timeout = 18000000 - block.timedtask.elapsedtime
</trial>

The above example computes the number of milliseconds remaining in the 5 minute period by subtracting the number of milliseconds that have elapsed since the start of the block from 18000000, which is the number of millisecond in a 5 minute interval.

There is a problem with this expression however, namely that it might return a negative number, which is not a valid timeout value, or it might return 0, which Inquisit interprets to mean no timeout at all. We can fix this by uses Inquisit's "max" function, which returns the maximum value of two expressions. The following ensures that the timeout is never set to a value less than 1 millisecond.

<trial timedtaska>
/ stimulustimes = [1=question]
/ validresponse = ("a", "b")
/ correctresponse = ("a")
/ timeout = max(18000000 - block.timedtask.elapsedtime, 1)
</trial>