Sorry, couple of questions to clarify things - what's the NPK read out on the side of the pack of your 'all purpose' plant food, and which Ficus do you mean (houseplant, fruiting fig?)

I bought a small bottle of " liquid plant food " that says multi-purpose formula , SCHULTZ brand and it has this no.10-15-10 which i don't even know the meaning ! My little FICUS is a houseplant bonsai one. Thank you in advance for your tips, appreciate it much !

Ha ha, it doesn't matter whether you know what the NPK stands for, but the NPK ratio you really want for your bonsai ficus is even and balanced, so 7-7-7 or 10-10-10, applied every couple of weeks during spring and early summer.

If you want to get really complicated, then one with a higher nitrogen content in spring, as growth begins (this will be the first figure in the NPK, the N part) is a good idea, but its not a good idea to keep using it throughout the growing season. Real aficionados of bonsai use a higher nitrogen feed in spring, a balanced one up till mid to late summer, then switch to a lower nitrogen one until feeding ceases as autumn arrives, but whether you want to bother with that is up to you. They're more likely to use this regime if the bonsai is an outdoor one, and particularly if its deciduous. That's only a general guide for non flowering bonsai plants though - those that flower need a different NPK ratio. Fortunately, yours is non flowering, is indoor and evergreen...