In period 2 you could apply the VSEPR theory etc etc, but in period 3 the bonding shell is that much further from the nucleus that VSEPR just does not apply.
Coupled to this is the fact that the methyl group is quite bulky compared to a simple atom and hence not easy to force to another methyl group- This fact would tend to widen the angle.. This all leads to the conclusion that I would expect a bond angle of based on 90º , but probably a little larger.
So let's look it up.
It seems to be about 99º from the only quick reference I could find (although it does not seem very reliable)