How to model & predict user activity/presence time in a website
I suggest approaches based on neural networks for time to event data. Depending on your data I think you can also use 

https://www.slideshare.net/mobile/datasciencelondon/survival-analysis-of-web-users

https://arxiv.org/abs/1807.04098

https://arxiv.org/abs/1801.05512

https://machinelearningmastery.com/multivariate-time-series-forecasting-lstms-keras/