Well, Ukrainian are rotating unit from frontline for training as we speak, this is going to drawn out for a long time.
Again, this is just 12 days (or 14 depends on when do you start counting) they don't need to hit Russian line now, in fact, you actually don't want to, you want to draw Russian up first, because the Russian would need to guess which way you are attacking. Ukraine, as an attacker, has the liberty to choose when and where they will launch their strike. Which mean it's ALMOST certain Russia would have to put reserve in first, the question is when.
On the other hand, unlike the Russian offensive which actually did failed after 5 months (Dec 2022 - May 2023) Russia is losing ground to the Ukrainian (I think they rolled back 8 village now) which mean the Russian is not really coping that well, look at the situation in reverse back when Russia is attacking, by day 12 back in Dec 2022, Ukraine had not lose one village at the same timeline, they started to lose ground back in March 2023.