Long enough. The development time varies with the developer. The recommended times from the manufacturer are usually close to the minimum that will give you a decent print. With a developer like Dektol, I generally give the print 1 minute 50 seconds. That is about 20 percent longer than the manufacturer's recommendation. The 90 second recommendation doesn't quite give me the results I want. At one time, with older papers, you had to worry about development fog if you developed too long, and the idea was to develop the print as long as possible before fog started to appear. You made tests to determine what that time was. Fog is not much of a concern with current papers, and you choose a time that gives full development. With warm tone papers, you can also trade off exposure and development time to some extent to vary the image tone a bit.