About B-2 extension

We plan to apply B-2 extension for my mom for two months. Please help some questions:

suppose

time T1: the time to file B2 extension;
time T2: the time when original I-94 expires
time T3: the time that the decision is made by USCIS about the B2 extension

My questions:

1. between T1 and T3: my mom is legal. She is in grace period, right?
2. usually how large is T3-T1? That is, how long does it take USCIS to make the decision?
3. When to apply for B-2 extension? My mom’s I-94 will expire in early April..

Any experience is highly appreciated.

Thanks a lot