Multi-homed implementation and BGP convergence time

[Andrew . Claybaugh]

Hello - my company currently has two connections with a single tier 1 ISP.
We are using the AS from our ISP at this time.  In the next month we will
be implementing a third connection with a second tier 1 ISP, so we will now
be using our own AS number on all three routers.  My question is when we
implement the new connection and update our existing connections to use are
own AS number, how much downtime will there be?  So far the second ISP has
only said that it could be hours for BGP to fully converge.  We are looking
for more detail about how long the outage will be and how widespread.

Will it be relatively short to our customers that are on one of the ISPs we
are directly connected to?  Is downtime less for customers on other tier 1
ISPs versus tier 2, etc. ISPs?

We will only be receiving a default route on each of the three connections.
Our routers will be advertising a small number of routes - 6 to 8.

Thank you.

Andy Claybaugh