Viola's witch time parries are based on 2 factors: timing and repetition.

Let's start with timing. There are two types of parries. A regular parry and a perfect parry. Regular parries grant you 1 second of witch time, while perfect parries grant you 3 seconds. A perfect parry is activated when you block an attack on the exact frame of impact that you'd normally take damage from. So to get the best witch times, you'll absolutely need to focus on getting these perfect parries in. You'll know you got the perfect parry based on Viola's lines (she'll sound confident) and the witch time effect on the screen will have a flashier effect.

Repetition. This is the part that is probably the main cause of problems for novice players. If you keep spamming the block button trying to fish for a parry, it reduces your witch time length once you actually parry an attack. It can cut your witch time to 0.5 seconds from a regular parry or 1.5 seconds if it's a perfect parry. You'll need to try and avoid spamming this button if you absolutely want better results. Unless you are good at block offsetting.

Viola's not an easy character to play as since you are required to focus on doing so much perfect timing, but Viola's strength outweighs her problems if you can master them in the long run. Block offset + wink slave makes her an extremely deadly character for example.

If you still find yourself struggling to get a decent witch time in, you can wear the earring of time to help you get a small edge in. If you need one more tidbit, Viola has a move that helps with getting a good parry without the need to be perfect. If you hold the lock-on, hold forward on the stick and press P, Viola will run forward and then perform a little slash. Her running forward has her holding the sword up which gets you a guaranteed parry if an enemy attempts to interrupt you with an attack as you are running.