True expertise on a large, complex code base does not just take a few months; competence is best modeled as growing over long timespans, not as something that quickly reaches an "expert"-level ceiling. This is all the more true when-- as is often the case-- the codebase is a complicated legacy thing, full of accumulated "technical debt" and unneeded complexity of all sorts. This is not to say that 100% pair programming is always the answer, but doing it to some extent is likely warranted.
This is exactly what I'm currently jumping into as a 'new' dev. I've been coding for about 3-4 years, am working on an ANCIENT php in-house ERP (we're still running v5!), and find the technical debt/unneeded complexity more shocking by the day. I've been in this position 3 months and feel like I've barely even scratched the surface of what is going on under the hood.
Teaming up for training is absolutely crucial at this time.
