Short answer. From the moment he converted the money to his own use. Article 1788 provides that a partner who takes an amount from the partnership coffers becomes liable for interest and damages beginning from the time he converted it to his own use, not from any later demand or discovery of the taking.
What the law says
The same rule applies to any amount he may have taken from the partnership coffers, and his liability shall begin from the time he converted the amount to his own use.
Civil Code, Article 1788 — Failure to Contribute Money. Read the full provision →
Liability starts at the moment of conversion
Article 1788 extends its rule on interest and damages beyond a partner's failure to contribute promised money: the same rule applies to any amount he may have taken from the partnership coffers, and his liability shall begin from the time he converted the amount to his own use. So for a partner who takes money from partnership cash and uses it personally, the interest and damages do not start when the partnership discovers the taking, or when it demands repayment; they start at the exact moment the partner converted the money to his own use.
Why the timing is fixed to the wrongful act itself
Pinning liability to the moment of conversion, rather than to a later demand, reflects how serious the law treats a partner helping himself to partnership funds. Unlike a partner who simply failed to hand over money he had not yet delivered, this partner actively took cash the partnership already had and diverted it for personal use. Because the wrongdoing is the act of conversion itself, that same act is what starts the clock on interest and damages, without needing the partnership to first notice or object before liability accrues.
What the partnership needs to establish
To pin down exactly how much interest and damages have accrued, the partnership generally needs to establish when the conversion actually occurred, since that date is what the law uses as the starting point rather than the date the taking was discovered. This can mean reconstructing the partnership's books or records around the time the funds went missing to identify the precise date the amount was taken and used personally, which may differ from when the shortfall was eventually noticed by the other partners.
What this means for the partner who took the money
Because liability under Article 1788 runs from the date of conversion regardless of demand, a partner who took partnership money cannot argue that interest should only apply from whenever the partnership formally asked for it back. The amount owed continues to include interest and any resulting damages for the entire period the money was diverted, and returning the principal alone does not settle the obligation if interest and damages accrued during that time remain unpaid.