dryer vents should be cleaned twice a year if you use the machine daily. If the vent was properly installed a small access cap should be provided near the floor just below the dryer hose connection. If not one should be installed. Most lint will fall to the bottom of the pipe. Ideally a pro should clean this pipe if you cannot access the roof. As the pipe goes higher and runs into an attic it is likely that dampness has occurred within the pipe and condensed. This causes the lint to stick to the pipe. A pro can use a chimney brush and run it up and down the pipe clearing any lint that may be trapped.
As far as the cap, There should be nothing other then a damper that opens and closes when the air blows. This should be already on the top of the roof. If not check with condo association to get them to fix it.
Also check with them, as many require a pro inspection and cleaning once per year. And they normally already have someone that has been contracted to do this .
This also depends on use habit and the type of "pipe" You can get some pointers from this article I wrote.