How Infonavit weighs on the calculation
The Infonavit deduction (between 5% and 30% of your integrated salary, depending on the type of credit) comes off your payroll before you ever see the money. The bank counts it as active debt: your available capacity is gross salary minus Infonavit minus taxes.
The room that's left
If Infonavit already eats 25% to 30% of your salary, banks tend to let you use another 10% to 15% on a personal loan. On a gross salary of 20,000 pesos a month with an active Infonavit, that works out to personal loans of 30,000 to 80,000 over 36 months. Aiming higher is hard, the model cuts you off.
Before you borrow more, think about consolidating
If you're already carrying Infonavit plus a maxed-out credit card and a store loan, a fourth credit isn't necessarily what you want. Consolidating the card and the store loan into a single personal loan at a lower CAT (Costo Anual Total — Mexico's all-in annual cost figure) frees up cash flow, and if you still need an extra personal loan after that, this time it fits.
If Infonavit is for your house and you want to buy a second one
Look at Cofinavit: you combine your Infonavit housing subaccount with a bank mortgage in a single transaction. It almost always works out better than taking a personal loan for the down payment and a separate mortgage on top, mainly because the mortgage CAT is far cheaper than the personal-loan CAT.