Any ideas on how far way I need to place the time portal from Into the Future?

You can simply put it in your household inventory if you want.

If you want to decrease the distance where you'll get the camera shake, you can do so with Retuner:

<FullClassName>Sims3.Gameplay.Objects.Miscellaneous.TimePortal,Sims3GameplayObjects</FullClassName>
    <FieldName>kCameraShakeDistanceRange</FieldName>
    <Index>0</Index>
    <Value>10</Value>
    <FullClassName>Sims3.Gameplay.Objects.Miscellaneous.TimePortal,Sims3GameplayObjects</FullClassName>
    <FieldName>kCameraShakeDistanceRange</FieldName>
    <Index>1</Index>
    <Value>100</Value>

I don't know exactly how far 100 is, but I would guess anywhere on your home lot is less, and somewhere not right next door is more.