Exodus 8:5

And Mosheh said to Pharoh, Glorify thyself on account of me. At what time dost thou request that I should pray for thee, and for thy servants, and for thy people, that the frogs may be destroyed from thee and from thy house, and be left only in the river?

L’MATHAI’ (AGAINST WHAT TIME) SHALL I ENTREAT FOR THEE? Rashi commented: “If the text said mathai (when) — [not l’mathai] — shall I entreat, it would signify ‘when shall I pray?’ But now that it said l’mathai, it means ‘Today I will pray for you that the frogs be destroyed by the time which you will set for me. Say then by what day you wish that they shall be destroyed.’”
In line with the plain meaning of Scripture, the removal of the plagues took place at the time of Moses’ prayer, as it is written, And Moses cried unto the Eternal, etc., and the Eternal did according to the word of Moses and it is not written “and the Eternal did so tomorrow.” The letter lamed in the word l’mathai is no proof that Moses prayed immediately, for the word l’mathai is equivalent to mathai, there being many verses where the lamed occurs [just for elegance of language], thus: ‘l’min’ (from) the day that thou didst go forth out of the land of Egypt; in the word l’min is superfluous. until ‘l’minchath’ (the offering) of the evening; in l’minchath is a superfluous style. the wing of the one cherub was five ‘l’amoth’ (cubits). in l’amoth is superfluous. There are many other such cases.

THAT THE FROGS BE DESTROYED. This is an allusion to their death, just as the expressions: that soul shall be cut off; and I will cut off from Ahab every man-child.
The intent of Moses’ repeating the promise to Pharaoh, And the frogs shall depart, etc., was to state that as soon as he will have prayed, the frogs will all be removed, and that Pharaoh should not fear that when these frogs die, others will come up from the river. Rather, the plague will be completely removed even though some of them will remain in the river. All this was to inform Pharaoh that the plague came from G-d for the sake of Israel alone.

התפאר עלי, arrogate to yourself the right to ask what you want of me and I shall carry out your wish. The construction is similar to Judges 7,2 פן יתפאר עלי ישראל לאמר ידי הושיעה לי, “lest Israel be arrogant enough vis-à-vis Me to say: ‘my hand has been strong enough to provide my own salvation.’”

למתי אעשה לך?, for which day and at what hour would you like for all the frogs to depart, and I will pray forthwith that they will all die before that deadline. Seeing that they do not normally all die at once, you will realise that my prayer will have been answered. [Moses wanted to forestall Pharaoh asking for the frogs to all die immediately. Ed.]

למתי אעתיר לך?. In order that you will recognise the difference between what your sorcerers are able to do and what G’d is capable of doing. When Moses offered two scenarios to Pharaoh, i.e. when the crocodiles would die, and henceforth restricting them to the river Nile, he also had in mind that whatever the sorcerers had done would be strictly limited in duration, and things would return to normal, whereas G’d could maintain the changes He had effected in nature indefinitely, totally or partially as He wished.