The frames of reference are:
1. "Gigachad" was 16 when he left.
2. He was framed a little under a month after he left.
3. The amount of time "Gigachad" spent in remand awaiting trial and then sentencing isn't stated.
4. "Gigachad" spent two years in Monte Ardente prison.
5. When "Gigachad" was released from prison, Miru already had a boyfriend. How long Miru stayed with this boyfriend before hooking up with "Gigachad" isn't stated.
6. Miru states in the present that she's been with "Gigachad" for "five years, three months and eleven days."
3. & 5. make calculating the exact amount of time since "Gigachad" absconded impossible. Added to that, "Gigachad's" current age is never stated.
If, for example, 3. and 5. total, say, 12 months, then he was away from Wollust for roughly 8.5 years.
Besides, "Gigachad's" relationship with Leia was NOT the reason why he left Wollust at age 16. If "Gigachad" and Leia did "something," it could've happened some time before he left, not immediately before. PS - I know many of us thought Leia was directly the reason why "Gigachad" left Wollust, but the real reason was set up in the original Chs 1-3, not just in the reworked Chs 1-4. It's likely only the minutiae behind the reason was altered.