I wouldn't say they are ungrammatical, but both sentences could be improved considering not only the typical word order in English but also how those words function in context.
The verb "practice" can be intransitive, but in the absence of an object and in the presence of a time phrase (esp. one that is not introduced by a preposition), the latter will tend to appear immediately after the verb.
1) Paul practices the piano for an hour every day.
2) Paul practices an hour every day for the piano recital.
Notice that in (2) the object "the piano" has disappeared and we may feel inclined to place the time phrase "an hour every day" after the verb.
There could be other alternatives, but I'd recast the sentence in question as follows (the main purpose being that it flows as easily as possible):
- Paul diligently practices an hour every day for his recital.