I talked to Linda and saw the "building relationship with Julie" I spent almost 2 weeks (in game time) spending time with her from morning till night time, yet Linda says the same.

For how long I need to keep building this relationship with Julie until I can proceed with the rest of the content?

Julie will need spending time with the various of activities that you can do with her. From what I know, these are what you can do with her:

First of all, the simple thing is just regularly talking to Julie after events. She normally has something new to say and what's coming next. If not:

Morning #1 - This is towards the beginning of the story, coming to the middle. You click on two optional scenes whether 'It's quiet' or take a peek.

Morning #2 - Later on in the game, Julie will be at the fridge grabbing some snack. The player can make her go back to Linda's room but you can have sex with her by the fridge.

Afternoon - In the afternoon, you can have the chance to join her with 'couples yoga' depending on how far in the story you are.

Evening - She will be sitting by the living room and you can normally get a quickie from her.

Night  - Depending at what stage you are at in the game, you are able to either watch a scary or romance film. Both of these can turn into a scene.

One more thing, remember to also exercise in Linda's room. That'll catch her attention. Hope this helps!