I know children would rather write in hiragana but what about for adults?
If 100% of your text consist of kana only without any kanji, that is 100% weird. Kanjis are used not only to help identify what meaning for for sound, but also kanjis work as visual dividers for text into small portions for better eye-brain parsing of text, because there is no spaces in Japanese text.
Is there any rule (of thumb) I can follow to decide it or is it just left to the writer's mood ?
There are cases, which are managed by some rule of thumb.
For example, here is a rule of thumb for two-rooted-verbs.
1) If the verb consists of two verbs and the first one is in -te form, the second verb is written in hiragana.
読んでみる [yon-de miru] not 読んで見る.
2) If the verb consists of two verbs and the first one is in -masu form, the second verb is written in kanji for its root and hiragana for its conjugation.
降り出す [fu-ri-da-su] not 降りだす and not ふりだす.
The real mastering of when to use kanji and when to use kana is a consequence of feeling it. After a year of dealing with Japanese language on daily basis, such feeling grows itself and there is no desire for any rules anymore.
For example, should I write こんばんは or 今晩は ?
To make a quick check:
go to Jisho.org.
For any word or phrase there is a mark "Usually written using kana alone". Using Jisho.org to check for [kanji or kana] is the "rule of thumb" for Japanese learners. But the final goal is to skip using Jisho.org and just feel it.
To make deeper checking:
go to tatoeba.org.
1) Type "今晩は" (use quotes to find exact match) to see examples of using with kanji and what phrases are used to translate this form (in your case it is [tonight], [this evening]).
2) Type "こんばんは" (use quotes to find exact match) to see examples of using with kana and what phrases are used to translate this form (in your case it is [Good evening!]).
3) Type "good evening", "tonight", "this evening" (use quotes to find exact match) to see examples of translation: kana or kanji (in your case: check if there is some usage of kana for [tonight], [this evening] and if there is some usage of kanji for [Good evening]).
Result of this deep checking for your case is:
1) They use kana to say [Good evening!]
2) They use kanji to say [tonight] or [this evening].