The Padres have had varying philosophies about when to call up their prospects over the past half-decade, so I feel like anything is possible.
However, I think the Padres will re-sign Giles -- even if they don't pick up the option -- to give Antonelli a tour of Portland. Personally, I think Antonelli could succeed now in MLB due to his plate discipline and hitting lines thus far. But, this is just his second season as a regular second-baseman (remember, he was drafted as a 3B) so his D may not be ready yet.
All told, I think he'll see time in the majors next season. Whether that's as the opening day starter (which I would pin at about 25%) or not remains to be seen. Also, don't forget that the Padres had no problem with Barfield being their major league starter at age 22. It wouldn't be entirely new if the Padres had a prospect straight from AA grace the lineup either. Headley was called up earlier this year straight from AA, and Jake Peavy came straight up from AA when he was originally called up.