长 can discribe time and length 久 can only discribe the time For example,"How often do you come here?"----你多久来这里一次？or you can say 你多长时间来这里一次？ "This road is too long!"----这条路太长了！

describing time : 长 equals 久 。all of them are meaning long time . 多久：how long I spend ? one week , one month and so on . 多长：what length is the rope ? one meter or more .

长describes the length of something 久describes the duration of certain activity or time frame. both can be used for describing time, but only 长can be used for describing the dimension of an object. you can also say 长久，meaning something has taken way too long to finish.