I already have a self sufficiency permanent residence card in Malta and wonder if there is any limit to the amount of time I can now spend elsewhere?
you can not be in OTHER country more that 183 days in theory to no be tax residence there that is for sure, becouse of most dual tax agreements have such sentence that if you live in other country more that 183 days you will become its tax payer
(Related question to this one here: Can Malta *permanent* residence be purchased? And Malta PR has what conditions )