Do you need those firstUrl and lastUrl aggregations to be terms aggregations? Because if you change those to min and max aggregations the solution is simple:
{
"query": {
"bool": {
"filter": {
"range": {
"page.time": {
"gte": "now-w",
"lte": "now"
}
}
}
}
},
"aggs": {
"sessionLengthData": {
"terms": {
"field": "sId.keyword",
"size": 10
},
"aggs": {
"firstUrl": {
"min": {
"field": "page.time"
}
},
"lastUrl": {
"max": {
"field": "page.time"
}
},
"sessionLength": {
"bucket_script": {
"buckets_path": {
"startTime": "firstUrl",
"endTime": "lastUrl"
},
"script": "params.endTime - params.startTime"
}
}
}
}
}
}