Hi CaroZ ,
I hope this helps, just update the code for number of text fields(in eq()) according to your requirement, the below code is for first 2 text fields. Make sure that you create the embedded variables (firstField, secondField etc..) above this question in the survey flow.
Qualtrics.SurveyEngine.addOnReady(function()
{
/*Place your JavaScript here to run when the page is fully displayed*/

//first field

jQuery('input[type="text"]').eq(0).keyup(function(){
starttime1 = new Date().getTime();
});

//second field

jQuery('input[type="text"]').eq(1).keyup(function(){
starttime2 = new Date().getTime();
});

});


Qualtrics.SurveyEngine.addOnPageSubmit(function()
{
/*Place your JavaScript here to run when the page is unloaded*/

//First field
endtime1 = new Date().getTime();
timeSpent1 = Math.floor((endtime1 - starttime1)/1000);
Qualtrics.SurveyEngine.setEmbeddedData("firstField",timeSpent1);

//Second field

endtime2 = new Date().getTime();
timeSpent2 = Math.floor((endtime2 - starttime2)/1000);
Qualtrics.SurveyEngine.setEmbeddedData("secondField",timeSpent2);
});