In order to benefit from SCA sanction and insurance coverage, any group that is not yet full status must have sponsorship from a full-status group for any event, demo, or other SCA activity other than a routine business meeting at which no combat occurs. The technical term for this is "administering branch", but it is often referred to as a "sponsoring branch" or "sponsor" for short.
Sponsorship can in some cases be for a specific activity, but more often it is a more lasting, mentoring relationship between your new group and an existing local group or barony. There are many benefits to this mentoring relationship, as you can imagine. Some kingdoms may mandate which group must be your sponsor, or they may let you negotiate with the group of your choice, but often the details of the groups' relationship to one another is left up to the two groups to work out. Generally speaking, a group is not required to sponsor a new branch, so it is very strongly in your best interest to establish cordial relationships with your neighbors from day one.
If your group is within or adjacent to a barony, you may request them to act as your sponsor. Again, this is voluntary on their part. A Barony could be your general sponsor, but you can still (with their consent) ask a local group to sponsor any particular activity.
The need for sponsorship is for your protection as well as the SCA's. For the SCA there is the obvious need to ensure that SCA rules are followed, to avoid liability or legal problems that might jeopardize our nonprofit status or cause lawsuits. For your benefit, by being an SCA sanctioned event your activity gains the protection of SCA insurance and also can be publicized in SCA newsletters and web sites. This requirement continues until you become a full status group.
A newly-forming group also cannot hold money in the name of the SCA and cannot have a checking account. However, your administering/sponsoring group can hold money for you in their checking account, simply earmarking the funds that are raised by and for your new group. If the group should fail, these funds remain the property of the SCA, but if your group becomes full status you can have that money transferred into your group's own checking account (still SCA property, but under your local control).
There are some things a newly-forming group cannot do, but there are many things you can do as well:
After some period of reasonable growth and stability as a new group, you can petition for advancement to full status. A full status group can have its own checking account and can hold events without sponsorship by another group.
There is NO FIXED TIME PERIOD for any of these stages -- it depends purely on your group's activity level, growth, stability, and demonstrated capability to resolve problems. When considering a group for advancement to the next stage, no one expects there to have been "no problems" -- instead, it is expected that people worked together to solve what problems did occur, and that they did so in a way that didn't break the SCA's organizational rules. It is likely to take several years to advance from founding to full status, so patience will be a needed virtue.