Mahaba ang panahon ng bisa.

Because in Filipino, the predicate often comes first.
In Mahaba ang panahon ng bisa, the descriptive part mahaba (long) is the predicate, and ang panahon ng bisa is the topic.

So the structure is roughly:

• Mahaba = is long
• ang panahon ng bisa = the period of validity

This word order is very normal in Filipino.

Filipino often does not use a separate copula like English is/are in simple descriptive sentences.

So:

• Mahaba ang panahon ng bisa.
  literally works like Long the period of validity.

But in natural English, you translate it as:

• The validity period is long.
• The period of validity is long.

This absence of is is completely normal in Filipino.

Ang marks the topic of the sentence.

In this sentence, ang marks:

• ang panahon ng bisa

This whole phrase is the thing being described as mahaba.

So:

• Mahaba = the description
• ang panahon ng bisa = the topic being described

A learner can think of ang here as introducing the main noun phrase: the period of validity.

Mahaba comes from the root haba, meaning length or longness.
As an adjective, mahaba means long.

It can describe physical length:

• Mahaba ang buhok niya. = Her hair is long.

It can also describe duration or time span:

• Mahaba ang bakasyon. = The vacation is long.

So in Mahaba ang panahon ng bisa, mahaba refers to a long duration, not physical length.

Panahon means time, period, or season, depending on context.

Here, panahon means period or duration.

Bisa means validity, effect, force, or effectiveness, depending on context. In this sentence, it refers to validity.

So:

• panahon ng bisa = period of validity
• more naturally in English: validity period

Here, ng links bisa to panahon and often corresponds to of in English.

So:

• panahon ng bisa = period of validity

This is a very common pattern in Filipino:

• araw ng pag-alis = day of departure
• oras ng klase = time of class
• dulo ng kalsada = end of the road

In this sentence, ng bisa specifies what kind of panahon it is.

Not only. Bisa has a broader meaning than just legal validity.

It can mean:

• effect
• force
• effectiveness
• validity

Depending on context, it may refer to something being legally in force, officially valid, or still effective.

So in a sentence like this, bisa sounds somewhat formal and can fit contexts such as documents, permits, agreements, warranties, or rules.

Yes, it is understandable and natural enough, especially in a formal or administrative context.

However, depending on the situation, Filipinos might also say things like:

• Mahaba ang bisa nito. = Its validity lasts a long time.
• Mahaba ang validity period. = mixed Filipino-English, also common in real life
• Matagal ang bisa nito. = It stays valid for a long time.

So your sentence is fine, but it sounds a bit formal and written.

Yes. That is also correct.

Compare:

• Mahaba ang panahon ng bisa.
• Ang panahon ng bisa ay mahaba.

Both mean the same thing.

The version with ay is often:

• more formal
• more topic-first
• sometimes more common in writing than casual speech

In everyday speech, many speakers prefer the first version:

• Mahaba ang panahon ng bisa.

Sometimes yes, but there is a nuance.

• mahaba = long in extent/length, including time span
• matagal = lasting a long time, taking a long time

For time expressions, both can sometimes work, but they do not always sound identical.

• Mahaba ang panahon ng bisa. focuses on the length of the validity period.
• Matagal ang bisa. focuses more on how long the validity lasts.

So matagal may sound more natural in some everyday contexts, while mahaba ang panahon ng bisa sounds more formally structured.

A helpful breakdown is:

• Mahaba = predicate adjective, long
• ang = topic marker
• panahon = period/time
• ng bisa = of validity

So the full structure is:

• [Predicate] Mahaba
• [Topic] ang panahon ng bisa

A very literal gloss would be:

• Long the period of validity.

But the natural English meaning is:

• The period of validity is long.
• The validity period is long.