So, I'm considering proposing a house rule to my group, so that Ritual Sorcery works as follows:

When performing Ritual Sorcery, the first step is to establish a link to the target. This is a Magic + Ritual Sorcery (Force, 1 Hour) Extended Test, with both the dice pool and the Threshold modified by the quality of the link. Thus, a material like provides no dice pool penalty or Threshold modifier, while a Symbolic Link provides a -6 dice pool penalty to this roll, and a +6 Threshold modifier.

Once the link has been established, the actual ritual may begin. This is also the point where the target may notice the magical energies building around them. Each hour, the entire team rolls their Magic + Ritual Sorcery skill in a teamwork test, building to a maximum of (Force + participants) hits. Each team member must resist Drain each hour until the ritual team leader decides to release the spell, which then affects the target at the total hits rolled.

How would such a rule affect ritual sorcery? How should I tone this up/down?

First off, the most important part of every house rule:
What's your intend by introducing said rule? Do you think ritual sorcery as written is too fast? Too weak? Too few dice rolls? Not fluffy enough? Something else entirely? You should never add, substract or change rules without a clear reason.
Without that information, I can only answer how I think about the changes where they introduced in my group, which doesn't necessarily help you any, as your prefered style of play may vary wildly.
Nevertheless, a subjective opinion.
The first roll is more or less pointless. It succeeds all the time and, at the worst, wastes a few hours of the ritual spellcaster with no drawback for taking longer, as the spell can't be noticed, so you may as well scratch it and save on the rolling.
As for the changes in the spellcasting itself, casting a spell for a single hour would basically yield the same effect as casting under the current rules, afterwards it strenghens the spell for a small amount of additional drain, making your way straight up better. I think ritual spellcasting is powerful enough already, so I don't see the need to buff it further.