Hi
I am very sorry. Please for ideally 60 ml in 24 hours (a minimum of 40 ml if you really struggle).
You adjust the frequency of feeds to how much you can get into Della in a session; it can range from having to fight for for every millilitre in the very weak. What you are replacing is not the veg and pellets (which make around 20% of the daily food intake but the hay/grass, which makes over three quarters of what a piggy is eating in a day. You can put 25 ml of water in a glass and see how little that is by yourself!
Vets not familiar with guinea pigs sadly tend to massively underestimate/ignore the crucial hay volume.
As close to 5-10 ml (plus additional water offered between feeds): every two hours during the day and once during the night).
10-15 ml (4-5 feeds plus extra water offered) between getting up und just before you go to sleep (make sure that you get 6 hours for yourself)
15 ml + (2-3 feeds daily during your waking hours), depending on how much she is starting to eat and drink on her own; manage transition by weighing first thing in the morning.
This guide here is also very helpful in understanding health monitoring and how to manage the various levels of syringe feeding:
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support
If Della starts to struggle against the syringe well in excess to her weakening strength or she struggles to swallow (please never give more than 0.1 ml or one tenth of a small syringe) to a very weak piggy and always wait until feed and water have gone down before giving more), then you may unfortunately face the fact that her body is closing down.
Not your fault; it can happen without warning in seemingly healthy piggies out of the blue at any age. You haven't missed anything because there wasn't anything you could miss.
I am sincerely wishing that the guide links below won't become necessary but here is more step by step guidance in case you need it if it comes to the very worst. A looming night and a weekend is always extra stressful. Please don't hesitate to contact an out-of-hours vet/24 hours clinic.
Is my guinea pig dying?
Emergency and Crisis Care as well as Bridging Care until a Vet Appointment
A Practical and Sensitive Guide to Dying, Terminal Illness and Euthanasia in Guinea Pigs
I am keeping my fingers very firmly crossed for Della and you!